What about images, links?
Formatted text like bold or underline?
I also prefer plain text, but in most of my emails I talk about technical stuff, or I send transactional emails that require actions, in which case showing buttons is a much better user experience than plain text.
Every MUA I've used allows the reader to set a font size, so changing font sizes is 100% a feature of plain-text emails. Then they get the link the size they need to read it correctly and it's absolutely easy to read. This here comment is pain text. Is it hard to read this link:
