You're acting like there's only two situations: The entire Fediverse defederates with them, or the entire Fediverse federates with them. That's not the case.

I, personally, do not want to interact with anyone using Threads, because Meta has a proven history of poor moderation and of manipulating the narrative for political gain on Facebook and I see no reason to think they won't do the same here. I am not the only one who holds this opinion. Those of us who feel this way can use instances that defederate with them, and have our way.

If you want to interact with them, you can maintain an account on an instance that does federate with them. You do not need to have a Threads account, nor does anyone else.