They won't at the start. But they will after they made friends on threads and then threads breaks the federation again. The people in threads will not move to the fediverse because most of the people they interact with are on threads. They might not even notice that the federation broke. But for the people on the fediverse it will feel as if a large part of their social circle just disappeared.
But before that threads will already make things worse for non threads users. They will start extending the protocol in such a way that fediverse servers won't be able to keep up. They will find undefined cases in the protocol and start using them causing other servers to break/degrade. They will start one-sided slowdown of federation causing people to blame the server they are on instead of threads. Etc.
More like https://en.wikipedia.org/wiki/Lingo_(American_game_show)