Google was only able to do that because XMPP's userbase was practically nonexistent by the time they pulled the plug. Google Talk had become such a hegemon that the overwhelming majority of XMPP interactions were between Google Talk users. Google had the option to keep tying themselves to a protocol that they don't own to support access to a very small amount of users or develop their own system that they could do whatever they want with. Lemmy and Mastodon do not have that issue. Regardless of what Facebook does with Threads, Mastodon has millions of users and Lemmy reached over 300k. The majority of their users are unlikely to migrate their accounts over just to sell their souls to the Zucc. There is enough activity on Lemmy and Mastodon that many Threads users may make seperate accounts here should Threads eventually defederate, while the users of Lemmy and Mastodon simply go back to an existence without Facebook's meddling. There's not a situation where Threads can destroy Lemmy or Mastodon like there was with XMPP