Users think of Mastodon as analogous to Twitter. You can tell them they are wrong. But telling new and eager users they are wrong, and then offering an elaborate explanation as to why they are wrong and then requiring them to make a confusing choice with unclear consequences before they can even use the service is not a great strategy for growth.

Are you kidding? The vast majority just use Gmail now, or perhaps an institutional account they were enrolled in “automatically” and have a strong motive to check regularly for school/work purposes.

Well, for the most part they don't, they use either their ISP e-mail, their work e-mail, or Gmail. iCloud might be a distant runner up. I rarely see anybody using even Yahoo or hotmail anymore.

In practice we don't see situations arising where Yahoo users can't send email to Gmail users because the boss of Gmail doesn't like that that the boss of Yahoo allows Fastmail users to send email to Yahoo users