Definitely was not expecting all that. They flipped the tables on us pretty brilliantly after last episode, when we were left thinking "Well, that's pretty cool. Kind of too easy in the end, and not really anything that different from the usual way the Big Bads get cleaned up."
That was just ... fucked.
As you said, Zamasu is pretty inarguably the worst (i.e. most successful) villain in franchise history now. He very well may have endangered the whole of the DB multiverse had Zen-Oh not stepped in like the Living Tribunal and deleted that reality.
And you're right: Trunks and Mai are the only survivors of their universe. The new timeline they went to might look just like their old one, but it's not their old one. It's just one that splintered off/was created when Beerus destroyed the other Zamasu.
I never expected this franchise to delve quite so much into the concept of alternate universes. It broached the topic in the most offhand way possible back during the Cell Saga, and then quickly moved away from it, seemingly without intention of ever addressing it again. And when "Dragon Ball Super" finally started fully exploring this route, I wasn't sure if I liked that they were doing it.
I do know now that I like it, but I'm also a tad confused as to the structure of the whole thing. Above, I compared Zen-Oh to Marvel's Living Tribunal, as that's pretty much what he is -- but there's only one Living Tribunal in Marvel's multiverse. He's omnipresent across all universes/timelines. That doesn't seem to be the case here?
They also can't seem to make up their mind whether the world Future Trunks comes from is supposed to be an alternate universe (i.e. one of the 12 universes known to exist previously), the future of the present day Z Warriors' world (Universe 7), or even the future of Universe 10.
We've seen drastic differences between the universes previously -- different deities and such. For example, Gowasu and Zamasu are supposed to be from Universe 10 while Universe 7 seemingly did not have either one of them. Instead, Universe 7 had figures like Grand Supreme Kai, Eastern Supreme Kai and North Kai (the role Zamasu had filled in his universe). But Future Trunks's world had most, if not all of them -- Gowasu, Zamasu, Eastern Supreme Kai, etc.
To elaborate: The anime only briefly mentions Eastern Supreme Kai in Future Trunks's world (when Future Trunks mentions that it was he who shared the information with him about Dabura's petrification attack), but the "Dragon Ball Super" manga shows Kibito and Eastern Supreme Kai die in battle fighting Dabura and Babidi alongside Future Trunks.
Anyway, Future Trunks's timeline apparently had a Beerus as well (an actual Beerus, not just an equivalent to him like Universe 6's Champa) who died when that timeline's Eastern Supreme Kai died (during the battle with Dabura and Babidi in the manga; during Zamasu/Goku Black's reign of terror in the anime). That's the implication of what Whis said in this latest episode -- that a new timeline splintered off when Universe 7's Beerus killed the present day Zamasu of Universe 10, and that he would then go inform this timeline's Beerus that he should seal Zamasu before that timeline's Eastern Supreme Kai died.
That then leaves us to wonder if Future Trunks's world is supposed to be the future of Universe 10, where it's apparent that all these figures existed together. This notion would seem to be reinforced by Beerus having expected Future Trunks's world to change when he destroyed the present day Zamasu of Universe 10.
However, this notion is also seemingly contradicted by the fact Goku Black (i.e. Zamasu of Universe 10 after killing his mentor and taking the Time Ring) traveled to Future Trunks's world/time period, eliminated Gowasu there as well, and then allied with the Zamasu there.
This confusion is still without even getting into how we can have two versions of Zen-Oh. The one who showed up in Future Trunks's world didn't even know who Goku was, so that rather definitively rules out this world being the future of Universe 7.
I just don't know what to do with this mess. A fun, awesome arc this was, don't get me wrong. I just don't know at all what to make of the metaphysics. If anybody can explain it to me, feel free.