Let me be frank: ELisp is a terrible programming language. I have a number of Emacs packages (in ELisp) and Sublime Text packages (in Python), and the development effort difference between the two is not even funny.
That said, Emacs relies rather heavily on unique features such as dynamic binding, advice, and autoloads. These are more or less impossible to replicate in modern languages, so I don't think there's a reasonable path to rewriting major parts of Emacs in, say, Javascript.
But we should look at what Lua did for Neovim. There has been an explosion of development effort now that Neovim finally added a reasonable programming interface. A similar thing could happen in Emacs.
That said, Emacs relies rather heavily on unique features such as dynamic binding, advice, and autoloads. These are more or less impossible to replicate in modern languages, so I don't think there's a reasonable path to rewriting major parts of Emacs in, say, Javascript.
But we should look at what Lua did for Neovim. There has been an explosion of development effort now that Neovim finally added a reasonable programming interface. A similar thing could happen in Emacs.