but I no longer believe that it is possible to build a competitive federated messenger at all.

The fact that we have a telephone system that works with separate providers contradicts this sentiment. If I want to pick up the phone and talk to my cousin's puppy in New Zealand, I can do that without creating an account on his provider's service.

I don't understand why we've forgotten this as a society. Yes, it was difficult to upgrade the phone systems over the past century, but it's worth it in my opinion. I really wish we'd start seeing government regulation that says "you should be able to talk to someone on a service without having to create an account on said service." I thought the DMA would do this, but sadly, Whatsapp still requires an account to talk to people using that service. Very disappointing.