I like the idea of separating the frontend and backend of a graphical app. But I feel like this is hardly a novel idea, maybe I'm missing something.

I take it you don't know about "X11Forwarding yes" or "html5 web app"

  For browsers, capabilities like connecting to Unix sockets have been dismissed as extremely niche

That is a security concern, that's why it isn't implemented. At least raw unix socks. You can have WebSockets and other ports only limited to http.