FWIW, I have been using ProtonMail for about four years for my personal email, and I haven't had any issues with communicating with folks from Gmail or Outlook.

Regardless, I don't think it's contradictory that people flock towards a somewhat-centralized system like Gmail; the point is that it still supports an open protocol. The fact that Protonmail "just works" when emailing Gmail is a testament to the fact that federation can work in practice, even if it does start becoming more centralized.