Ecosystem yes, and also it's architecture and culture. ELisp I would say no. If we ignore the specific flavor, then 99% of what's realistically done with with elisp, is also covered by other dynamic languages like JavaScript. And the rest is so esoteric that it's probably not that worth the trouble.
> I don't see how supplementing ELisp with JS would make Emacs more attractive;
Not everyone likes lisp, not everyone is willing to learn a new strange language for unknown benefits. And JS has a vast ecosystem of its own. Far bigger even that Emacs heritage, but on different areas. A well working combination of both, with no side making significant sacrifices, could be very alluring for many more people than Emacs has now.