This sounds like something you could have proposed 15 years ago. It assumes local, custom email clients for "enabled-by-default encryption" and XMPP support. People have moved onto web and mobile email clients, and you can only use whatever is compatible across all platforms.
I also think it overlooks mobile adoption. Many people using the internet today have only accessed it from a phone. They don't necessarily have email addresses. They may not have ever used a full sized keyboard.
This idea does not preclude the use of a mobile app. The mobile app will simply act as a IMAP and XMPP client on the network side, and look like a Facebook/Google+/MySpace app on the user side.
I also think it overlooks mobile adoption. Many people using the internet today have only accessed it from a phone. They don't necessarily have email addresses. They may not have ever used a full sized keyboard.