Emacs is written in C and Emacs Lisp. I’m not able to download the source right now, but from some googling the C portion is around 20-25%. So 75-80% lisp. That’s a rather substantial amount of code that would need to be rewritten, not to mention that probably does not include all the lisp code that’s not included in the core distribution. Tons of packages out there that would need to be rewritten.

  -----ADDED-----

Downloaded 27.2:

  Emacs Lisp: 1,171,507
  C         :   318,611
  C Headers :    43,738

(that's without comments)

That's a lot of code to replace, and again, just what's in the core distribution.