I would argue that is the point of federation. Despite the fact that most of the users are on gmail, they can still email anyone anywhere else, without issues.

I would argue that is the point of federation. Despite the fact that most of the users are on gmail, they can still email anyone anywhere else, without issues. And I've never heard of one of the big name providers blocking emails from another large provider, with the possible exception of spam heavy domains. It's a lot better than social media. I feel like the analogous situation would be being able to follow someone's twitter feed in your FB feed, or send a twitter DM to someone's facebook messenger account, and how likely is any major social media platform to allow that to happen? Despite the concentration of power, email's federation makes it much easier to choose which provider you want to use, avoiding the network effects that come from walled garden services.