This already exists: the browser is the shared runtime. What needs to happen is everyone shipping Electron should instead ship a local HTTP server and open the user's default browser to run the UI. You can hide the browser chrome and make real top level windows by making your app a PWA. Anything else that needs to happen outside of the browser sandbox can be handled by the local server.

The only disadvantage is you lose control over the browser version that's running, but you also lose that with a shared runtime (unless every app pins their own runtime version which defeats the purpose of sharing). Besides, most Electron apps also have a web version that has to support all browsers anyway. The advantage is you seriously reduce the disk and memory footprint of your app and shed the responsibility of shipping prompt patches for Chromium security issues.