XMPP, famously, died when Google dominated the network and then defederated. Leaving XMPP users no choice but to sign up with Google to keep in touch with the bulk of google users who were no longer there.
Threads does not need to embrace to extinguish, it has 1.6b accounts ready to activate and now over 100m activated in the first three days. Universal defederation will drive a lot of Fedi-users to Threads simply because, if you’ve come from sites with hundreds of millions of users (Twitter and Reddit), it’s going to be very difficult for many to recreate the breadth and depth of content here, and certainly not as quickly.
The Fediverse would technically survive, as XMPP did. But it would likely shrink, not grow.