I'm just talking about requiring SSL/TLS for every SMTP connection between mail clients, servers, and routers and encrypting datastores.
Most* of the time this does happen in the real world so, y'know.
Perhaps less than you think. I used to work for a small community bank here in the States, and one of my jobs was testing and verifying this very encryption between the bank and its service providers (to prevent the chance of sending a customer's sensitive data unencrypted.)
I saw about a 75% encryption rate**. The most glaring exception? Google. Every time a message went through Google -- even when they accepted it via a TLS-encrypted connection, it was shunted between several of their servers in plaintext, then passed on. They used encryption into their network and out from their network but not internally.
Now this was a few years ago, so maybe they've fixed this, but I learned then not to assume that just because the technology is widely available, that it's widely in use.
**Google's Transparency Report includes a section on this: https://www.google.com/transparencyreport/saferemail/ and their current numbers are roughly in line with what I found two years ago.