I’ve thought about this a bit before and I don’t hate the idea. The benefit of using a monospaced font is that it’s a quick indication of “this isn’t a WYSIWYG editor” and further distinguishes the editing space from the preview in the composer.
There are some downsides though; monospaced fonts do take a bit longer to read and have some general quirks. IA Writer has spun these as benefits, but kind of caved and developed their own new “kind-of monospace” typeface to avoid those quirks.
Either way, there aren’t any significant problems this solves with our existing editor, and it’s pretty easy for admins to customize this themselves on their instance - so any sort of additional feature wouldn’t be high priority.