Quoting the linked email:

  So if we go for Guile-Emacs, we'll be stuck with Guile, i.e. we'd
  have (old and new) packages that use Elisp, new packages that use
  Scheme, maybe yet other new packages that use, say, Javascript (or some
  other language support by Guile).  That would make the work of Emacs
  (and GNU ELPA) maintenance harder.