Emacs Lisp's Future
Given all the reworks of software projects that have a) been started b) then delayed c) then delayed more, "because this time we'll make it perfect" and d) then failed to gather traction because of incompatibilities, it's my humble guess that the only way would be to continue ELisp as it is and slowly rework the compiler/interpreter and the runtime into a more modern codebase, and add missing features such as thread/process based concurrency. Guile, Clojure, Common Lisp or whatever language are going to basically restart Emacs from scratch, no matter how well they're coated with compatibility macros.It's better to create slight incompatibilities gradually (so that packages can catch up in some humane timeframe). I could imagine the ELisp runtime being intertwined with the Emacs C code badly enough that making radical changes to it would seem "impossible", just like in the CPython codebase it is considered impossible to ditch GIL and modernise the code. But those kind of comments are often excuses because people are lazy and it's nicer to write new code.