Hemlock is not a reimplementation of GNU Emacs; it's derived from earlier Emacsen, just as GNU Emacs is. Hemlock in particular is based on ZWEI, and was in version 0.99 around the time that work started on the GNU Emacs project. It's a slightly older sibling of GNU Emacs, rather than a descendant of it.
GNU Emacs has undoubtedly become the de facto standard Emacs, but I don't see why that disqualifies other branches of the Emacs family tree from using the name that all of them inherit from the venerable TECO implementation.
My point was that Emacsen have already been built in Common Lisp, but that they do not serve as substitutes for GNU Emacs--a point with which you appear to agree.
Maybe we even agree on the reason why: because no Emacs implementation can substitute for GNU Emacs without supporting its ecosystem. The amount of work needed to do that would be enormous, whether the proposed substitute is written in Common Lisp or in something else.
As you said, the value lies primarily in the ecosystem. Common Lisp is by far the better language, but GNU Emacs is by far the most empowering/practical environment. I don't think enormous work would be needed, there are plenty of powerful shortcuts one could take (some ideas are in this thread) and Emacs Lisp can be made to run on top of Common Lisp but it's safe to say that for whatever reasons the will/intent is simply not there.
Emacs Lisp is what we have to go on with and it's nice to see that it is not stagnating.
So far, nobody -that I know of- has tried to move GNU Emacs to Common Lisp. GNU Emacs is a well-known, well-used platform and moving to Common Lisp could very well be worth the upfront costs.
Wishful thinking aside, the GNU Emacs development community has hunkered down behind Emacs Lisp, so improving it will bring immediate and tangible benefits to every GNU Emacs user. I love Common Lisp and do use SBCL for a lot of personal projects, but I also love GNU Emacs and I will take any improvements there that I can get. Watching Emacs Lisp become a more viable general purpose language (improved performance means an expanded set of problems that it can now address) is a great development. Stefan Monnier on emacs-devel: