Afaik, whenever an Activitypub instance has defederated from another it has always had to do with some combination of bad user behavior, poor moderation, and/or spam. Are the various instance admins who have decided to preemptively block threads.net simply convinced that these traits will be inevitable with it? Is it more of a symbolic move, because we all hate Meta? Or is the idea to just maintain a barrier (albeit a porous one) between us and the part of the Internet inhabited by our chuddy relatives?
(For my part, I’m working on setting up my own Lemmy and/or Pixelfed instance(s) and I do not currently intend to defederate.)
A few years ago, Reddit strategically banned two terrible subreddits during one of their shitstorms (the whole AMA firing scandal), just as Voat was getting popular. It turned from a decent community that was starting to grow and challenge Reddit’s presence, to a right-wing extremist cesspool overnight.
You also see this sort of thing happen to subreddits all the time. Some of them go from a good community, and either slowly or quickly, shifts into a much more terrible version of itself. Russian bots/ops have transformed subs to push their own agenda.
The community matters, and how that community evolves matters a great deal. Communities can live or die from massive migrations like this.