While I fully, really, completely get all the arguments on every side around Meta implementing ActivityPub, I'll confess to not understanding the profound concern that this will kill the fediverse.
Federated social media never "belonged" to anyone. Feel free to block Threads. Build disparate communities!
Or federate with them!
The network and power dynamics are not & will not be any way worse than they are now, or before Threaderation, or before Musk bought Twitter. They are only better*.