More to the point, emacs already has been rewritten substantially. There’s the initial rewrite from TECO to lisp, there’s the 80s implementations like uemacs (famously used by Linus), we had a major fork with xemacs in the 90s, there’s guile emacs (ready any day now), lem is almost but not quite a Common Lisp emacs, and it’s been making a lot of progress recently. So go ahead with your rewrite, you’re in fine company!