Defederating “from this and that” is actually sometimes problematic here. It’s about instance admins finding balance between freedom and usability (limiting spam and hate). Beehaw.org defederated from lemmy.world and sh.itjust.works, lemmy.world defederated from exploding-heads.com etc. These decisions were controversial, but they weren’t bold. On the contrary, much thought and care went into these and that can be seen in communities’ support for them (in case of Beehaw, along with hopeful awaiting of refederation by users and admins alike).
But that seems not to be the main issue you’re presenting. Defederating from Threads specifically is an entirely different matter. And people who advocate for it, including myself, have more arguments for it than just privacy.
Though it's not the main point of my comment, I'm gonna list some such arguments, simply to back my words.- The EEE. Meta could (and quite probably will) try to federate with its millions of users, then use extended protocols putting pressure on Fediverse to adapt, in order to satisfy Meta’s users. They can make it difficult to keep up (e.g. by providing purposely flawed documentation) and the users will grow tired of stuff not working here but working there. Once users register with Meta (since it’s a part of the Fediverse after all, right?), they’ll cut the rest of us loose. - Badly moderated content. Facebook is already full of it. - Meta has a history of terrible actions and should not be supported.