What would stop that from repeating? Well, even if Threads abandoned ActivityPub down the line, where we would end up is exactly where we are now. XMPP did not exist on its own outside of nerd circles, while ActivityPub enjoys the support and brand recognition of Mastodon.

The whole point of EEE is to leave the original project in a messy state. With most of the fediverse userbase being "Meta Threads" users, developing anything for other fediverse software or making it somehow incompatible with Threads is not a (good) option. With a decent chunk of software and side projects being only compatible with the Threads "extended" (and likely undocumented) version of ActivityPub, Meta still has the ability to pull the plug on all of those. A lot of Mastodon users will become used to having interactions with friends on Threads, and will be forced to switch over by the time Meta kills federation.

It's not just "a doom and gloom post", it's a valid concern that the communities and environments built up over years will get killed off, with a known strategy for doing so. Why else would Meta (Facebook) suddenly play nice with federated platforms, when their history is to buy out or kill off competitors?