OG purism doesn't really... excuse the more ardent shipping insanity. The natural conclusion to these characters has always been "Cloud and Tifa end up together in a post game world" or "everyone is dead anyway." Those are the only material realities that are plausible and make sense for these characters.
If it's the latter, then no one is having a relationship with anyone. They're all intermingling in some weird polycule of mishmashed personhood. Seriously though, what does romantic preference even matter at that point? Everyone is just lifestream goo. The universe does a lot of reflection on death, but it's still pretty frank about what it is. We and the characters are meant to understand that there is finality to death, because life is a precious thing that needs to be cherished.
Anything to suggest C/A only really has merit in like an AU sense. Which I mean like... I guess the OG kinda leads the audience to consider? But I think the game is looking at much bigger things when it does. It's asking the audience if we can collectively imagine a better world. Perhaps in that world, loss can have meaning beyond the empty feeling of grief. Maybe that world feels a lot less doomed than the one you're living in, and maybe that means you can better come to terms with things like death and grief.
Even if you argue that's inclusive of some romantic look on C/A (to which I'd probably shrug and say, yeah sure, if you want), it's not exactly the biggest takeaway. I don't think the story is posing these kinds of questions to make people wonder about how bad Cloud wants to smash his maybe-would-be-at-one-time girlfriend. I really don't think that this aspect of the story is for the expressed purpose of building some grand romantic narrative between Cloud and Aerith where their love transcends time and space. The story simply is not really about that. The OG is intentionally open ended for a reason, and I reject that those reasons have anything to do with who ends up with who.