Meh. Users have no issue picking an email provider. They can pay with money (eg proton) or with their data (Google, Hotmail), champion their local identity (think gmx.de) and they manage to pick the one that works for them. Why should Mastodon's diversity be an additional burden?

In fact a lot of people are now on Mastodon because they picked Instagram. Soon another platform may follow suit and federate. I'm sure users will understand pretty quickly that they can communicate cross platform just like a Gmail user can mail a Hotmail one.

It's really not that hard to grok.