Very good article, thanks!
There are some minor points to add:
1. [Privacy] Many organizations (companys, administrations, etc.) still have their own email servers. I.e. this area is not yet centralized completely.
2. [Censorship resistance] Many users, incl. non-geeks, have multiple email/XMPP/etc. accounts. I.e. even without nomadic identiites a service outage can be survived.