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).
Neither am I. But universal pre-emptive defederation just cuts to the end game without any kind of fight. Meta users won't even notice if/when they defederate because they never knew about us in the first place. And defederated instances will lose users to Meta because some people use social media in ways that only work well with bigger networks.
I'm all for some instances saying they want their networks to stay small and users who prefer it that way should have somewhere to go. But users who want a bigger network should have better options than signing up with Meta.