In practice, what we see happen with the big private Matrix federations is that the users start demanding being able to talk with users on other private federat

In practice, what we see happen with the big private Matrix federations is that the users start demanding being able to talk with users on other private federations, and/or the public Matrix network - and threaten to otherwise start using WhatsApp or Telegram or whatever for these sensitive but external conversations. So there is a huge incentive to actually federate properly, and we're talking to pretty much all of them on figuring out how to do so.