That's still a very different class of problem from a CVE break of a single-point-of-failure is the point. Someone decides to use your Electron app's old Chromi

That's still a very different class of problem from a CVE break of a single-point-of-failure is the point. Someone decides to use your Electron app's old Chromium build and a known CVE in it as a target vector for infecting or remote controlling user's machines is a very different problem from "because we are using system webviews this small CSS/JS feature doesn't work in this version of macOS, please upgrade". Even if the user rightfully blames you the app developer for both issues and doesn't allow you to point fingers upstream, one is a severe security threat that can destroy entire brands and the other is "mere tarnish" on a brand. (I know which class of problem I'd rather deal with, given the choice here.)