What you eventually get is a single global list that the majority of instances use
Not necessarily. Defederating too many instances means that your own instance will get less content; admins know that, so good admins generally avoid doing it unless necessary for the goals of their instances. Couple that with dissenting points (for example: grotesque but morally acceptable content, porn, dumb/low-quality content…), and the odds of said “single global list” popping up becomes fairly small.
Instead I expect to see a bunch of smaller lists, between instances with similar goals, and plenty unilateral subscribing (e.g. A subscribes to B, but B doesn’t subscribe to A).
From what I understand, there are already instances who operate this way.
That’s good to know. If they do it automatically, this system could be already implemented across Lemmy.