There's no such thing as "lisp all the way down" unless you're running a lisp machine. Common Lisp, e.g. SBCL, is written in C. So I'd say Emacs already is almost "Lisp all the way down" in any meaningful interpretation. Elisp is written in C, yes. And there are a few functions written in C for performance. What abstract benefit is worth completely replacing this with some other language that will inevitably be written in C anyway?

In the meantime, people are actually improving elisp, giving us things like the new async options and performance improvements. People like to hate on elisp because of some ugly syntax and some missing stuff that most mainstream programming languages have, but it's uniquely crafted for Emacs stuff and fills that role better than any other language.