We are siphoning users off corporate platforms already. Almost everyone on the fediverse has left those networks to come here. They did it without Facebook or twitter federation, and people will continue to do that without Facebook or twitter federation.
Threads absolutely not “reshape” itself to fit the fediverse. They are playing to win, and winning for them means mass collection of analytics and social graphs to sell micro-targeted ads to their customers.
The fediverse is growing at the right speed, IMO, and inviting Facebook in to play, in the hopes of luring a few users is a bargain with the devil.