My own opinion has come around to match Eli's (far more relevant/expert) on this topic, but in case it helps anyone else: I think this reflexive desire to keep a strong hold on all the existing elisp comes from the long line of emacs clones like Edwin, Hemlock, climacs, Alpha, and a long list of less well-known alternatives. I "lived through" several of these, and the sense I got from every one I actually tried was that people eventually fell back to the "real thing" emacs, largely because it worked (well enough?) and already had some facilities that were missing in the putative replacement. 

summary: I think it's hard to unlearn this lesson from history

It's entirely possible that the shift of "typical computing" towards massively multi-core distributed etc. is the final straw, or at least the high bar that a massively-shared-state-lisp-machine can't vault -- while still being good enough to cross most of the moats we see in everyday usage. It's also possible that there's some adaptation that will arise, perhaps along the lines of how web workers/service worker threads interact with the DOM in the modern browser, that keeps Emacs going even longer. I remember the days when "Yeah, that will happen shortly after the release of Emacs 21" was the in-joke for porcine aeronautics, and we just saw Emacs 29 released, so: it could happen.

I do think that a very early step needs to be "figure out how to handle concurrent analysis and editing across multiple cores and perhaps machines", but that probably just reflects a bunch of my personal interests.

~Chad