You say: The people who are at currently at this point have already gone to Threads. Then you say that if traffic from Threads is subsequently withdrawn, all the people who haven't already gone to Threads will... go to Threads.
You are basing it on the idea that Threads federating is a temporary move designed to advertise Threads. It's a theory. But seems unlikely. If Threads goes away again, I suspect that the current Fediverse userbase will, by and large still be here.
Blakerboy777