Lack of ability to distinguish between pre-formatted text and regular prose alone makes it a complete non-starter for most who aren't reading monospaced text in a terminal. I don't really like reading monospace text for prose (many people don't), and using proportional fonts means things that are supposed to be aligned will break (and if you restrict yourself to text only, you'll find yourself aligning things on occasion for tables or other content).

I actually use plain text in FastMail because it's "better" than HTML (usually), but it's not good.

Ability to send text/markdown natively would be pretty brilliant.

Ctrl-D to format the selected text as monospace in the Fastmail composer is pretty nice - I use it often for sending bits of log line or terminal output when writing up incidents or describing how to do something.

RTF would be fine, then.

Yes, it would be. It is a pity HTML won over RTF.

plain-text with some sort of simple formatting would be fine (markdown/asciidoc). We don't have to go overboard in the opposite direction...

Gemtext[1] would have been perfect and way enough while still being legible on a terminal. Italic and bold can be done via unicode, you don't need a markup language for that.

[1] https://geminiprotocol.net/docs/gemtext.gmi