The story of how Google “killed” XMPP is a popular just-so story, but it’s not as clear cut as is so commonly described and isn’t directly applicable to ActivityPub. It’s not the same situation. XMPP is an instant-messaging protocol, which was very dependent on network effects - you had to adopt Google’s changes if you wanted to be able to talk to your friends, and when Google eventually ditched XMPP support (apparently because external servers were the source of a tidal wave of spam they didn’t want to deal with) that meant everyone was cut off from Google’s users and that was bad.

Whereas if you look around the general opinion of the Fediverse, it sounds like being cut off from Threads’ content would be seen as a positive thing by most. There’s no need for complete interoperability in the Fediverse, there are already plenty of servers that refuse to talk to each other even though they share a protocol. If Threads proposes some change to ActivityPub that nobody else wants to use and Threads gets cut off as a result, how bad will that really be?