To be more popular Emacs needs to be easier to get started with and easier to extend. If I have to read a whole book or learn an entire new programming language, I'm not going to put the time in. If any time I search for "how to do x", I get a disjointed mess of "but spacemacs...", "in prelude...", "this custom elisp snippet..." where every piece of advice can be potentially incompatible with any past or future modif…
Emacs would benefit from being easier to get started with? Yes. Emacs is held back by not being easy enough to extend, because it uses Emacs Lisp? I don't see how. I'd expect learning to write plugins for some application to be harder than figuring out enough syntax/semantics of a language to call library functions.
In any case, learning Lisp is useful, even if you never write an Emacs plugin.