How would they extinguish though? By proposing changes to the protocol that smaller instances can’t implement? At the moment if you didn’t want to be in Threads you just don’t have to be.
Google pulled this off with XMPP by having their Google Groups bridge be horribly maintained, feature incomplete, and randomly go down for days at a time. Most of the people on XMPP were on Google Groups so to them it just looked like the few people who actually hosted their own XMPP servers randomly went offline. It got to the point where people who used XMPP would have to create an account on Google Groups in order to reliably be able to talk to their friends. Google Groups users eventually came to the conclusion that it was XMPP that was unreliable.
By having many more users than the rest of the fediverse combined, basically becoming the default twitter replacement most new users flock to. They'd be taking up all the available "oxygen" as Mastodon withers away and dies.
If just 1/100 of their users who they've automatically given accounts use Threads regularly, there's a big risk of that happening.
idk if it can, because a lot of ppl who are currently on mastodon, are there because they wanted to get away from platforms like twitter and threads. these users will stay no matter what and would prefer not being able to see the content from threads or whatever other corporate social media might join the fediverse, over switching to those instances.