Emacs, on the other hand, has some pretty big system dependencies (like gtk3, alsa and imagemagick on a modern linux) and one step of compilation where it actua

Emacs, on the other hand, has some pretty big system dependencies (like gtk3, alsa and imagemagick on a modern linux) and one step of compilation where it actually loads up all the elisp stuff (which is really slow to load), then dumps the memory holding the loaded elisp stuff, and on next startup it just reloads the dumped state into memory. Somehow I strongly doubt that's going to be a simple "cargo install foo".