If messages can be sent in plaintext, they will be sent in plaintext.
Email is end-to-end unencrypted  by default. The foundations of electronic mail are plaintext. All mainstream email software expects plaintext. In meaningful ways, the Internet email system is simply designed not to be encrypted.
The clearest example of this problem is something every user of encrypted email has seen: the inevitable unencrypted reply. In any group of people exchanging encrypted emails, someone will eventually manage to reply in plaintext, usually with a quoted copy of the entire chain of email attached. This is tolerated, because most people who encrypt emails are LARPing. But in the real world, it’s an irrevocable disaster.