> It sometimes needs to be told on a paragraph-by-paragraph basis, because otherwise it might also reflow ASCII art diagrams and such

Yes exactly, there is no "one right way" to do it because you never know if a newline is a "hard enter" or just a "reflow to the next line". Every default will break something.

This is why you need some (minimal) markup, no matter what you do text-only email is broken for some use cases.