I don’t feel like this is a 1-1 comparison with the XMPP issue.
With XMPP users joined servers and then Google started working with the protocol allowing those users contact books to increase by hundreds overnight. Then when Google dropped, many people built work connections through those chats and still wanted to talk so they migrated to Google as it was the simpler platform.
But fediverse isn’t about talking to individuals for the most part. Mastadon gets the closest, but even then it’s about following updates and quickly getting ideas out to large groups, not the 1-1 communication of a chat. Fediverse is mostly about talking in communities. When threads defederates after getting a bunch of users, those users will lose entire communities instead of just a couple individuals that can switch platforms. And asking a community of hundreds to move to your preferred social isn’t how it works. People go towards communities, not companies. When threads defederates (which will happen) they will lose members, many communities will drop in membership, but I think the fediverse will be in the same spot after that it would be in if threads never existed. Not the same as today. But if we ignore threads and try to project the growth out 5 years. That’s where I think we’ll be if threads defederates in 5 years.