Mastodon communities are already coordinating trying to fully defederate threads. This is definitely going to be interesting to watch as a big whale jumps into the fediverse with the intent to profit
That's why nobody takes the 'fediverse' seriously. Threads is not the first server being blocked because moderator x don't like the owner of that server. Imagine if signing up for a @gmail.com email meant that you can't send or receive emails from @outlook.com addresses because the owner is mad at Microsoft. They could have taken this as a big opportunity to make ActivityPub mainstream but nah, let's keep gatekeeping…
Just curious, what do you mean by "mainstream?" Did you really mean widespread adoption of ActivityPub (AP) or did you mean "one additional instance of an AP instance that has millions of users?"
Meta getting into the Fediverse doesn't do anything to make AP "mainstream," rather it exposes the existing users of the Fediverse to a single instance that has the users that many of those in the Fediverse setup/joined instances to get away from.
What's more, Meta's business model (hyping conflict and clickbait to drive engagement and ad revenue) is antithetical to many in the fediverse.
As such, it's not surprising that many admins don't want anything to do with them, as they've shown themselves to be bad actors on their own platform. And that's the main reason admins ban instances -- because they're (at least from the admin's point of view) bad actors.
It's almost enough to get me to federate my AP instances just so I can ban/block Meta. But I don't even want to federate with extant AP instances, so I won't bother.