Sorry, I can't even feign concern about the state of Vim's source code. I'm a Vim user and plugin developer, not a Vim developer.
A single-threaded UI means I can easily tell what Vim is doing at any time. It means the editor never feels clunky because indexing or some other background activity is hogging resources. It also means I never get a "process is running" prompt like in IntelliJ or Emacs when I want to exit.