I think it has more to do with the way Mastodon decentralised and I imagine the people who figure out to do e-mail as a platform first, will be the next twitter.
The big issue with Mastodon is that your "twitter handle" depends on the server you join, which makes it hard to find you. If it was just your e-mail address, however, then everyone would be able to find you, and keep finding you even if you decided to change servers.
You would be able to change servers and keep your username. Mastodon isn't really decentralised, it's just twitter with more twitter companies.
SoMe should be email based, and servers should just be relays. That way you could also create private SoMes for things that you'd do in Facebook group by simply setting up a relay on something cheap.
Some hosts/domains even mirror Mastodon addresses and email addresses. There is nothing stopping an address from being both, especially if you control your own domain.
Your e-mail address isn't tied to a server. It can be, but it doesn't have to be. I've had many e-mail services without changing my e-mail address.
I can use g-suite, fastmail, outlook for business and a range of other things, and the people sending me mails won't notice. I can't swap my Mastodon server.
The big issue with Mastodon is that your "twitter handle" depends on the server you join, which makes it hard to find you. If it was just your e-mail address, however, then everyone would be able to find you, and keep finding you even if you decided to change servers.