GNU has had two common Lisp implementations under their umbrella at least since mid 90s (CLISP (of Viaweb fame) and GCL (of Maxima fame)), but due to RMS' hate of Symbolics and Common Lisp, all of the effort was invested into making Guile run properly (which took them 25 years and still nobody is using it) and the two Lisps were basically orphaned.
The latest effort of guile-ifying Emacs is more pushed by the Guile guys, in order to attach themselves to an already existing killer app (emacs) and to basically force more and more people to use Scheme, because nobody is doing it voluntarily.
The Emacs guys, as you see from the post, are not _really_ excited about the idea of switching to another _language_. They want FFI, multithreading, etc, but they do not necessarily want Scheme. Actually they dont want Scheme at all, adopting Scheme would be just a price to get FFI, etc.
Of course the proper way would be to go Common Lisp. Elisp is and has always been a subset of Common Lisp, and they've had a Common Lisp compatibility layer for decades, and a lot of packages use it. The problem is simply highly political. You'd have to confront RMS, and you'd have to finally get rid of Guile's "official GNU extension language" status.