That's the monoculture.
Edit: didn't realise you were responding to someone refuting what you had said, haha. so basically we agree, I just thought you were someone else.
And while iOS has the greatest market share for mobile, you're right, WebKit doesn't have the greatest market share for general browsers. That's definitely Chromium, now on Windows, macOS, Linux, iOS, Android, your mum's toaster, just about every 'native' web app…
Side note: I guess we should really be saying Blink, that's the name of the engine. But even if, say, you use Google Chrome on iOS, backed by WebKit, that usually makes you a Google Chrome, backed by Blink, user on the desktop.