They are unscalable. I love Emacs, and it continues to be my editor of choice, but it doesn't seem the design is amenable to adding good threading primitives.

They are unscalable. I love Emacs, and it continues to be my editor of choice, but it doesn't seem the design is amenable to adding good threading primitives. NeoVIM seems like a success story in this regard, though VIMScript is a much less pleasant extension language than Elisp, which is not perfect, but is at least a real programming language. There may be a way forward for Emacs, building a sandbox that looks like a full Emacs instance to existing Elisp, and slowly factoring out the whole-editor blocking issues; but that might be almost as complicated as starting fresh, with fewer benefits.