So if Captain Marvel is around, she goes. If she isn't, the movie better say so. Same for superheroes like Dr. Strange or Thor.

Yeah, I do agree that one of the big issues hamstringing the MCU going forward is that it's getting less and less plausible to have these standalone cataclysmic events without other superheroes getting involved. Some throwaway one-liner about 'here's why we can't call the Avengers for help' may have cut it in Phase 2, but after Civil War, Infinity War / Endgame, No Way Home, and various other lesser instances of crossover, it's much harder to make excuses for keeping other big names out of a crisis on the scale of Tiamut bursting out of the Earth's crust.