Seriously, WebKit on Mac and Edge on Windows would be just fine for >99% of Electron apps. Using the available web runtime would be a tremendous saving both in build size and memory usage.
Did you know that Chromium contains 25.3 million lines of code? [1] Each Electron app is bundling a separate copy of that enormous codebase in binary form. It's a terrible waste of memory.
It's true that supporting the native web runtimes would be more work for the Electron framework developers, and sometimes would also mean more work for app developers... But most web apps have absolutely no good reason to be tied to Chromium, so for app developers the diversity would actually be beneficial in the long term IMO.
If someone wants to start a project that's like Electron but uses the native runtimes, I'd actually like to contribute some work. Let's call it "Electrino"?
[1] https://twitter.com/worrydream/status/857794605076500480