The only way they co-opt the existing userbase is if everyone defederates from them and people who need/want a bigger network have no option but to move to Threads. This is what happened to XMPP and we risk doing it to ourselves this time around.

I'm not saying no instance should defederate. There are good reasons to avoid them. But if there are no independent instances federated with them, Meta dominates the space by default and without anywhere else for its users to go (unless they want a smaller network and know about the existence of defederated instances).