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.

> Are the big editors unsuccessful?

No, but if that's the only criteria, we already have plenty of those.

My first test of any text editor is to open something like 4-gigabytes log file with one gigabyte line. If editor works fast, it's good. So far very few editors pass this test, so most are not suitable for general use, only for some niche use like editing tiny text files.