If they defederate from other instances, they just means Threads users won’t see those instances. Those instances will still see Threads content, if they want. The content is also shared across instances this way, so their servers largely don’t matter. Whenever Lemmy.World or Yiffit.net is down or having problems, I just bop over to Kbin and it’s like those other two instances never actually dropped out since I can still see and interact with their posts.
I don’t see how in any way shape or form Threads can or will fuck up the entire fediverse when even if they have a majority of the users, their content gets spread around the whole network and doesn’t stay on shit they control.
And if you’re worried about their app collecting data: then don’t fucking use it. Unless you think their app, on someone else’s phone, will collect YOUR data somehow, this is a completely bullshit argument.