@anthropy
I think you're looking at this from a "fediverse vs other platforms" angle whereas I'm talking about "federation on the fediverse vs federation elsewhere".
If I create an email address, it's simple: I choose a provider by comparing some straightforward data points such as storage, cost and privacy. Then, joining communities is a separate task where I then check the rules, vibes etc. Providers and communities are completely separate.
If you join fedi, your instance is provider and community at once. The latter is true even if you don't like it, because this whole network decided to treat instances this way.
"All of the reply guys are from mastodon.social" "Yeah I'm blocking the whole domain lol". An admin doesn't like your behavior and suspends you from their (remote) instance, which cuts you off from everyone on there, which is a community decision. You will get caught up in this dynamic.
Have you heard of dansup blocking people from Loops for using a certain email provider as someone he disliked? It's the same core behavior, but in that case it was called out as stupid because email providers don't have the same role as fedi instances.
You brought up other platforms, but your comparison doesn't work with my angle, so here's my take:
With fedi, the community aspect is tightly integrated and there's no way around it.
With email, communities aren't part of it at all. They happen out-of-band and use email for notifications and personal messages at most.
This means that unlike with fedi, what happens in those communities won't affect your email account at all. You will never lose your ability to exchange emails with someone.
As a consequence, email federation is so inconspicuous that most people just automatically understand that they can send a message from Gmail and someone using Outlook will receive it.
Compare this with fedi, where you have to find out how much of the network is blocking an instance, if you can even talk with certain people, if you vibe with the rules and people on there etc.