It’s not that they would necessarily cripple it, but they would “enhance” their instance of AP (the “extend” in EEE), “accidentally” making it incompatible with the rest of the Fediverse and thus creating an excuse to suddenly drop support for the Fediverse. At this point users in, say, Mastodon will have created some degree of dependency on users in Threads, and at that point people in there would be forced to move to Threads if they want to maintain a similar experience as before.

Can I ask you why do these examples not apply in your opinion? The XMPP one seems particularly fitting and actually maps perfectly to the Fediverse situation (so far), if I’m not missing anything.

Yes. And there are good reasons for that, as this user showed: https://lemmy.world/comment/884169