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?
Facebook can bootstrap their product with federated content made by users who are in the fediverse because they don’t want to support a company like Facebook. By not defederating, you would be helping Facebook every time you post a comment or make a post because you would be giving Facebook free content to further their for-profit goals.
Facebook will also be taking fediverse content and displaying it next to ads.
Defederation is one-way. An instance you defederated with still has access to your instances posts.