What you'd need is node (without webkit) which translates to ~9MB, + launching with the system browser.
9MB or so would still be a large step down from entire chromium runtime, but desktop apps still will have access to things like fs module, which are needed for desktop apps.
I made stubs for "path", "url", "process" so that the Electron Hello World can be loaded. Doing "fs" next seems like a good idea, but I'd like to have a real-world app to test it against to see which APIs actually get hit (rather than just reimplementing all of "fs").
I saw your comment yesterday and it really gave me something to think about. I swear if you don't go down this path I will as soon as I get a chance!
Ideally developers would have a spectrum of choice: all native libraries (Electrino), native browser + Node (your suggestion), Chromium + Node (Electron). They could then pick what works best for the scale of the project.
I can see how the architecture is greatly simplified, and I can still imagine using something like this... but giving up npm is a lot.
Re: npm, you can of course still use an npm-based workflow for the browser-side part of the app (Webpack, whatever). It's just the Node-side services that are missing.