The issue is the importance of all the existing packages — invalidate them and nobody will use your implementation.

There are small syntactic differences between Emacs lisp and Common Lisp so a lexer that handled those would suffice.

There are some small semantic differences but they could be handled by porting code. And the Emacs c code could be rewritten in lisp or ported into the Common Lisp implementation, depending on your taste.

The existing package ecosystem is one of the main value propositions of Emacs. Otherwise a re-implementation with a different extension language could have replaced it a long time ago.

> There are some small semantic differences but they could be handled by porting code. I suspect there are more than just small differences. And someone would have to do the work to port the code. I suspect that if packages that people use don't work, they will just avoid using the new version of Emacs, as opposed to abandoning the package.

It's not an impossible project, but it's not clear to me it's easier/better than the proposed solution.

The point of my suggestion was to not have to rewrite any existing .el files.