On the flip side, if we federate freely with them the result is the same (and probably worse).
First of all, if you can access all federated resources (I think it's more aimed at mastadon, and connections with the lemmy/kbin will be limited. But I've not looked at what threads is offering yet) then we've already seen what people as a whole do. Just check the flagship instances lemmy.world and kbin.social. People just flocked to the biggest servers. Well, now the biggest server will be threads.
But then the existing users. They will have friends saying "Oh you know, I signed up on threads it's great" and they can say oh, well I'm already on mastadon and you can add me at [email protected]. And now they're connected.
Since the majority of new users will be on threads, not most connections users have are on threads.
So, if and when it turns out Meta remove federation for whatever reason (there are so many they can choose from) the people still on mastadon instances (and maybe lemmy/kbin/etc to a lesser extent) most of their friends are gone. Now the fediverse people will stay. But normal users? They'll sign up for threads and move on.
I'm not convinced there's a winning route once they're in. But, maybe I'm just the pessimist.