Heh, what's funny about the string theory inspiration is that (if I understand correctly) it suggests other universes would be fundamentally different which would make any similarities impossible. So it's a close enough excuse for bizarre cosmic zones like comic book prisons or video game special stages, but not somewhere you'd find another Earth or whatever. I will credit Into the Spider-Verse for making atomic incompatibility its pressuring plot point though!

Anyhow, the trope itself isn't the issue (rarely is, right?) as much as the fact that it's being abused to throw AU characters at us without telling their stories, which doesn't work either way:
• If they're similar and we're expected to assume that they're almost the same person, then like you said, it takes away the stakes because there's always still a bazillion other scenarios where they're okay/not okay.
• If they're different, how can we possibly care when we don't know anything about them? And even if we do, e.g. bringing back legacy incarnations, they're still usually not that relevant to the current incarnation's environment and just end up being fanservicey background noise.

This brand of apathy is all over Bayonetta 3. Not just the otherworldly characters, but the central ones too barely have any substance and just fight their way through hollowly stylish setpieces and gags—as if any actual story was just written to fill gaps between levels and trademark mainstays.

I could forgive all this if the gameplay was a big improvement, but even that takes more steps backward than forward. Seamless as the summons may be, and as fun as the playstyles are on their own, they come at the cost of enemy variety or any cool HUGE bosses which was the best thing about Bayo before. Why introduce new features that require you to give up everything else that was special?

Nice! Now if only there were any good movies…

What's most peculiar is that a bunch of licenses have jumped onto a "multiverse" craze simultaneously, and unlike the preceding universe trend (which was clearly a Marvel-inspired fad), this one seems to have sprung up everywhere by coincidence?

Like, I swear even the Tomb Raider game director hinted at it last year by specifically saying the aim for the next game is to "unify these timelines." He might have meant mechanically, but either way, the phrasing shows how much buzz there is for that trope at this point.