Came here to mention the same thing. It is incredibly surprising to see that they've not improved the usability of plain text after all these years. Arial is still the default font both for composing and reading.

Now, what I'd also really love to see is a 80 character width guide while composing plain text emails. That may be too nerdy though, but I'd just love to get as close to my usual Vim/mutt config as possible.

Maybe they'll at least get a lab feature out for us plain text users. At least let us customize our plain text font choices. For now, there are a few Chrome extensions that fix this issue.