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I still see lots of different emails out there, outlook/hotmail is still huge, yahoo occasionally, icloud in the US.
Among my techy friend circle all of us have either our own self hosted mail, a ‘privacy’ company email, or something in the middle.
All to say, I don’t think it’s that uphill of a battle for the very large percentage of Internet users to accept the way federation works.
At what level? I get a student email from my college (outlook based) as do the professors, though communication is primarily through Canvas. So that’s what I see most often in that context.
I think a lot of people have Gmail incidentally for things like YouTube and other Google account stuff, very few people know you can even bring your own mail.