The whole point is that you CAN'T separate the two of them. Guts has to be able to choose between his revenge and keeping his son alive. There isn't a world where he can have both. That's because this is a mirror to the way that things have always been with Griffith, even going back to what Miura put in the manga almost 20 years ago:
The whole journey in Berserk is just Guts going through the pain that he's put others through without knowing it. As the only member of the Band of the Hawk who Griffith explicitly chose, rather than those who decided to follow out of charisma, he was the only one who wasn't signed up to pledge everything he had to Griffith's cause — which was blood and death. When Guts decided to leave to pursue his own dream, he made the same moral decision that Griffith did, but just from a place of ignorance rather than one of understanding.
That's why when Griffith first re-emerges as Femto, his demonic armor is a literal bird head that opens it's mouth and stretches around him the way that Guts' Berserker Armor does. Griffith is always keeping that power in check, because what the story is following is a heightened metaphor for the type of people they are. Guts has always let his ego get lost and consumed in combat where he doesn't have to think and can just operate on instinct, whereas Griffith always has to work to constantly keep those impulses in check all the time, because they will cripple his dream if he gives in to them.
This is an underlying reminder that Griffith has ALWAYS gone out of his way to avoid harming Guts whenever possible. He always asks Guts if he's willing to do something rather than just outright ordering him to as a commander. Even prior to the Eclipse, he tries to run away and kill himself to keep Casca from staying with him out of pity. That's when he saw a future where he'd given up that dream forever and lived with Casca and their child, while Guys was still of somewhere getting to have the dream that he'd abandoned them for. Griffith can't speak and tries to tell Guts not to keep running to save him, because by doing that, he's invalidating all of the reasons that they had to go through hell. That's why he makes the choice in the Eclipse, because it is the path to victory with the least total bloodshed, and it guarantees that everything wasn't just a sacrifice for nothing. Episode 83 makes that even more clear with Griffith himself thinking that the idea of causality and fate in a world that dictates those things out of pain and fear is the very Idea of Evil, and he's ALWAYS felt no qualms about using the power of corrupt kings who are obsessed with him for his own ends as just a stone that happened to be in his path.
That's the ignorance that Guts shows, because he believes that he is that, rather than the only person Griffith ever chose to be by his side. The only thing he wanted was to be able to take off that armor and get to be a kid around Guts where he felt safe, like having a family. That's the whole purpose of the Band of the Hawk and even moreso of Griffith's dream of a kingdom, and when he has it, why he overtly talks about ways to make immigrants and refugees who have no home and are from other lands to have a clear path to a sense of belonging and trust.
The whole struggle throughout the series is that Guts never realizes what he has until AFTER he's already thrown it away, and then he has to experience the extremity of that pain from the other side. Everything in Berserk is a reflection of the hurt that Guts causes others and him slowly losing more and more of that ignorance, and trying to determine what the correct path through impossible answers is with full clarity of what the choices of his actions mean — which is what Griffith has been doing the entire time, and why he operates out of necessity when closing off his emotions to love or hatred that he felt when he has to make a terrible decision, or even when he loses himself to that emotion and sleeps with the princess of Midland or Femto rapes Casca the way Guts did when taken over by the Beast of Darkness.
The purpose of those opposites is to go into constantly deeper detail about how they are exactly the same, but also why, while they long to have one another, once that link between them is broken, the pain becomes unbearable and is often permanently dangerous to try to put back together. (which you can see in resolutions like Naruto being Hokage and Sasuke being forced to protect Konoha from outside of the village). Another good examination of those themes is in Tekkonkinkreet where White's suffering is because of his deep awareness of everything that Black is experiencing, despite seeming like a child. It's about how Black seems like he's the one protecting White, when in reality it's the other way around (and that manga/anime sets up a lot that's in Naruto). Even moreso are all of the Judeo-Christian themes in Mamoru Oshii's Angel's Egg, which was made just after he lost his faith in Christianity, and it's essentially the Biblical flood narrative and waiting for the bird that can find dry land after the apocalypse, and the end reveals that all of the unborn Angels are trapped in unhatched and twisted, embryonic forms that are exactly what BERSERK uses for Griffith's design as Femto, and why he has always had the Egg of the King.
Griffith's destiny doesn't allow him the luxury of ignorance and being a carefree child who just gets to curl up in someone's arms and forget the world around him. That's why he takes those moments whenever he can, and after choosing Guts, he has someone who knows the same suffering and who he can trust to protect him from anyone else ever harming him. That's why he's happy and content — because Guts has always been his OPPOSITE rather than his equal. However, Guts' ignorance to what that means when he hears Griffith talking about it is what catalyzes everything that comes after. It's why the only harm he can't protect Griffith from is harm that Guts himself causes. That's why when they met at the Hill of Swords, he always tells Guts that nothing has ever changed. It's why he doesn't bear Rickert ill will for slapping him in the face, and actively tells those around him not to retaliate. Griffith know that will just fuel more justification for retribution, which leads to more death, which is what his path is full of and why he focuses so heavily on the means by which he achieved those ends.
As the Moonlight Boy, he's still the same as always. It is the dream of a life that's been lost to him forever because of Fate — the Idea of Evil, and why he's usurping that power to try and bring about a better world. They're the same as the opposing paths that Nausicaä & Kushana get placed on in the end of the manga version of Nausicaä of the Valley of the Wind. All of these manga are predicated around the core underlying existential examination that these two individuals are forces like Yin & Yang, and that so long as there is conflict between them, it will only ever amplify hatred. There is no death that will ever bring about justice. The hatred that Guts has for Griffith is exactly like the hatred that the little girl hanging from his sword has when she swears to kill him for killing her father, the Slug Count at the beginning of the manga. While he might accept that that FEELS like justice, Guts' death will just cause pain to all of those who love him and rely upon him for protection and assistance, the way that he still has to rely upon the Berserker Armor to be able to hold a sword in this chapter.
The biggest message in Berserk is all about how many horrors befall the world out of ignorance, and how much suffering it takes before being able to understand that revenge never brings justice or peace, only selfish catharsis on an attempt to soothe the pains that you experienced as a child and inherited from your parents that you're now in a position where you can choose to continue to perpetuate and reflect those horrors into your own child, or you can find a way to force yourself to change to break that cycle — which is the primary goal that Guts has been pursuing, even while he's never yet learned to recognize what it is BEFORE he'd already thrown it away. That's why the Moonlight Boy being Griffith is the culmination of all of that development and how he needs to choose his path forward, because there is no having it both ways.
Also, this is why the visual design, historical references, and mythological integration of those concepts in BERSERK has been a key component of what I've been writing for the last year or so, as they are always about Yin & Yang as a mix of a small amount of light in the dark & vise versa. If you want the clearest example of why — the finale of Advent Children after Cloud's Omnislash v.5 leaves Sephiroth with a single Angel Wing (for the first time in the FF7 series proper) matching Griffith's Incarnate form, hovering above Cloud — who is standing surrounded by all of his fusion swords embedded in the ground around him, representing all of the comrades, friends, and loved ones he's lost exactly like the Hill of Swords in Berserk where Guts & Griffith finally meet up again. The difference between them is that Cloud is glowing in light because he won't let go of that darkness within him (Sephiroth) while Sephiroth is wrapped in a black feathered wings, because he won't let go of the light within him (Cloud), and even more firmly reiterating those parallels. In BERSERK that's even more layered:
Guts inner self clings to the fear that he is a curse and an omen of death, so he's striving to be good by learning that if he can trust others that can be a role he takes to protect others, but that's why his emotions and fears getting the better of him represent the Berserker Armor that wrap around him that takes the shape and form of the Beast of Darkness. Griffith is always afraid he'll never get to be a carefree child surrounded by love, so he's striving to be the commander who creates that for everyone else so that he has an environment to occasionally relax in, but giving in to that weakness is what makes him irrational and emotional and ended up with him becoming Femto, which is why he is wrapped in his Incarnate form as the Falcon of Light.
For this chapter, the Moonlight Boy's black hair being that in Griffith is just like the white streak in Guts' own hair — he can't just undo the events that put it there. Life is about finding a way forward to keep the things he cares about safe while also learning how to not further amplify those hurts — because they are tolls taken from both sides every time, even if he only sees them from his own limited perspective.
That's why this chapter breaking that illusion of comforting ignorance is about as perfect of an ending for BERSERK that I can think of in the absence of ever seeing Miura's execution of the story's conclusion. It honestly feels like it ends almost exactly where Miyazaki's Nausicaä and Tezuka's Phoenix did. Just a stopping point in the story that you know keeps going.
X