I came to the Threadiverse because Reddit was closing its APIs and building the walls higher around its garden. I will be supremely disappointed if the Threadiverse collectively turns around and does the same thing.
So instances on the fediverse have some obligation to let entities who (A) will control 99% of the content, against our values of a decentralized, more evenly distributed fediverse; (B) have zero interest in an open fediverse; and (C) have all the incentive in the world to prevent its growth and get more people on their own platform to ensure profit? As usually hesitant as I am about preemptive defederation, if the fediverse is to preserve its values of openness and ensure its growth, it can't let in for-profit corporations that will control most of the activity and that go directly against those values of openness we care about so much. Just as tolerance doesn't mean letting in those who are intolerant, an unwalled fediverse can and should put its guards up against those who want to take everything for themselves.
it gives Meta no incentive to do things right
Meta already has zero incentive to do things right. In fact, they have negative incentive, as people being on Mastodon or Kbin instead of Threads actively harms them. You will never see Mark Zuckerberg suggest that people spread out to other instances so that no one gains too much control, but you will see him try to get as many people from the other instances on Threads as possible. We are talking about making our activity dependent on a for-profit tech corporation. If we were way larger so that Threads wouldn't control such a massive portion of activity, I wouldn't be as concerned, but as things stand now, we're letting our content pool be dominated by a company that has interests in direct opposition with ours. I can't see a scenario where any of this ends well.