yeah that’s true currently but only because Google spent the past 20 years undercutting every other email service such that only professionals, nerds, and professional nerds ever send emails outside of their own domain, and everyone else balks if you try to end a dictated email address with something other than “at gee mail dot com”
same goes for websites, and in both cases the solution is not avoiding trying it, it’s education and access
which is not to say there aren’t dozens of other problems with federation but this isn’t the biggest of them