I'm no fan of Meta or their practices, to be clear. Though I do think there are potential benefits in having the ability to communicate cross-platform, so long as some reasonable safeguards are put into place. I'm firmly in the camp that doesn't believe that Google killed XMPP because XMPP was never a popular or widespread protocol prior to GTalk, and the users who came and went when Google did what they did were Google users, rather than XMPP users. So much like Eugen here says, it went back to how it was before they got involved.
That said, I do think that Meta in its current incarnation is an entirely different animal. I suspect that early on in a post-federated world, we'll start seeing dark patterns intended to lure users to Threads. I'm envisioning registration gates similar to paywalls on news sites. "This content is available exclusively on Threads! Click here to register your account!" type stuff. More sinister, there's nothing to stop Meta from appending advertisements in the body of posts created on Threads. Hell - they could go full evil genius and suppress that they're doing it entirely on their own platform since they'll have some other ad delivery mechanism there, which would mean the only people being served those ads would be federated users OFF of Threads who see or interact with content created on Threads.
So while I'm not a doomsayer about Threads and federation, I do think that we as a community are going to have to make some decisions about how to handle them. Having access to a community the scale that Meta will produce isn't necessarily a bad thing. And because of how Lemmy / Mastodon / KBin / Fediverse apps work, we as users will always have the ability to control what we see in our feeds. At worst, it makes /All/ less usable, which is admittedly quite a big loss given how useful it has been to get subscribed to worthwhile content since joining Lemmy. And obviously, some instances will elect not to federate with Threads at all, which gives users choice on the type of community and content they want to interact with regularly.
With some care, likely some effort around defining usage rights for user generated content, and some new content control / filtering capabilities yet to be developed, I think that these networks can coexist in a way that is mutually beneficial to a degree, but if not - defederation is a click away.