> I don't think Electron is required in order to have rich extensions - just see Vim/Neovim, Emacs, or any IDE

At the cost of difficulty. Recently I wrote a VSCode extension myself with zero prior knowledge (shameless plug: https://github.com/CrendKing/universal-format-on-type/), in probably one day. All I needed to study is their extension API that I cared about. No need for learning a whole new language or DSL. Also easy to debug since there is no compilation and little environment setup.

There is a good reason even though everyone acknowledge about Electron's performance cost, everyone is still using it.