Even if Firefox and Chrome used Gecko and Blink with JSC, that's still a marked improvement from the state of things today. Most of what I dislike about Safari are its weird rendering issues, which would be solved.
For example, I use Firefox as my main browser, and the lack of e.g. proper WebAuthN support on macOS has a much, much higher impact to me (and is making me consider switching), whereas I couldn't care less about e.g. WebUSB (which I do occasionally use, but I'm fine switching to Chrome for that use case).
To get an accurate picture, you'd have to somehow weigh all of these various standards and features in a way that not only accurately presents actual users, but also their relative importance (i.e. do websites trying to use something fail gracefully, or do I effectively have to use another browser to get achieve what I'm there for).
Some apps work around this by running JavaScript in an invisible WebKit view, but that approach has IPC overhead, so is probably only worth it for processes that interact with the UI and rest of the app infrequently (which is the opposite of what you‘d want from a JS engine interacting with the DOM).
a-Shell uses it for performant WASM execution of CLI executables, for example.