As much as I love Markdown, I can't see it totally replacing TinyMCE (or any other WYSIWYG editor WordPress might use in its place). Even something as simple as a link with target="new" must still be done in HTML, so you still end up mixing markup with Markdown. And not everybody knows HTML (I'd like to see some data on the percentage of WordPress users' knowledge of it, honestly), and that's where the editors still have to come into play.