I think calling any of the Marvel movies science fiction, and evaluating their world building in SF terms, is probably a category error and definitely setting oneself up for disappointment.

Because, like, all the world building is crap. This isn't new or isolated to Eternals, it's always been crap, and it will always be crap because the movies do not care about consequences and ripple effects and systems. The only things that matter are the immediate nonsense plot and the ongoing character arcs of the super people in this particular movie.

That's it. It's universe-impacting scale scoped down to the emotional and world building horizons of a movie about two people getting divorced. If you're really invested in whether John or Jane gets the nice china the huge stakes make it feel super epic. If you're not, it makes it feel shallow and narcissistic.