Supporting server-to-server ActivityPub is slightly risky for them as it gives people an alternative, and there will always be outward bleed. On the other hand I think it takes a lot of pressure off potential legal troubles around being a monopoly (which they must be worried about if they actually manage to kill Twitter).

I predict if they manage to be the overwhelmingly dominant platform in the Fediverse, they’ll keep it working. If they see another big competitor jumping in and taking a lot of users (or hell, if Mastodon starts to eat their lunch), I’m not so sure.