The hardest part for maintainers would be saying no when everyone inevitably asks to expand the feature-set.
That said, I suspect that many of the more modern JS APIs which are organised as separate modules are much easier to exclude when building something like Electron. I'm talking about these kinds of APIs: https://developer.mozilla.org/en-US/docs/Web/API
Many APIs there to exposed things from deep inside the rendering engine, but there are still plenty left which are likely separate, like Bluetooth, Gamepad, Ambient Light, Beacon, IndexedDB, Image Capture, Geolocation, Payment request, Web Audio, etc.
Builds of Electron with exotic APIs removed could be good although I'm sceptical of whether that would bring the run-time memory use down much, i.e. what (API) you don't use, didn't hurt you anyway.
Anything else is just the laziness of not caring about Web standards and shipping an whole browser instead.
Every day ChromeOS is one step closer to overtaking all desktops.