It feels as though Dr Erin MacDonald has earned her consultant’s fee helping them sort out the physics.
We’re out of the mess of the ever-expanding manifold time that Marvel and DC have bought into.
Beyond that version of infinitely branching manifold time / multiverse being offside, basically contradicting modern physics, it creates a situation where every possibility exists so nothing our heroes do matters, and nothing ever done to fix a time incursion matters either.
Instead we see that forks in the timeline can prune other timelines. Both branches can’t continue to exist, and the river of the timeline has some fixed events (time crystals) that pin it, pull it back to its original course.
So it would take something extraordinary, even on the Trek scale of extraordinary, to create a true ongoing branch. The creation of the Kelvin universe is associated with the Romulan Supernova. Knowing now that the Romulans have been interfering with human development over centuries and using temporal agents to do it, having a major disaster to the Romulans impact human history seems like a corollary. With a major event like a sun blowing up, we can say we’ve got a threshold for creating a separate sustainable universe.
As for TNG Parallels, I still love the episode, but perhaps we could reframe it as all the short run alternate timelines. For as unlikely as it was, Worf got back to his own timeline and Enterprise. Time fought back.