Could you flesh out the "federation doesn't scale" argument a bit?
> So while it’s nice that I’m able to host my own email, that’s also the reason why my email isn’t end-to-end encrypted, and probably never will be. By contrast, WhatsApp was able to introduce end-to-end encryption to over a billion users with a single software update. So long as federation means stasis while centralization means movement, federated protocols are going to have trouble existing in a software climate that demands movement as it does today.
Contrast with WhatsApp where they control the entire stack from client to network to server. They can push a single set of updates and everyone has the new hotness.
Stage one is that the old email servers become an end for all of their users. From there services can start shifting to having clients where users control the keys.
>Every client would need to support whatever E2E encryption scheme chosen.
If someone couldn't use gmail with an email client the author of that client would have a big incentive to add support for it else risk their users switching to a client that does.