So wait, you’re suggesting that Discourse take 2 months to overhaul an existing project? Just so that people have more choice of text editor?.. (It may take less time, because Markdown takes HTML, but – WYSIWYG editors unanimously produce ugly HTML, and it will be tough for any communities to transition to any other forum software, past, present, or future, if the posts are in HTML.)
As @sam said, “people can create plugins.” From looking at the code, I can vouch that it will be possible to completely overhaul the editor in a plugin.
Maybe a better route would be to work towards getting those extensions that exist for other editors built for Discourse.