A bit of forced diversity is wildly preferable over another collapse towards monoculture.
Says who? I certainly don't agree with that statement, the "web as an open platform" was unrecognizable when proprietary clients were the norm.
(And before you say it, consumers can choose Android, where 99.9% of people use Chrome, but apparently that's okay because they "chose" it.)
WebKit is a second-class citizen on the web, and we can argue about why that is, but nobody can sincerely deny it. Apple's strategy has been dead in the water for over a decade. iOS lost.
Meanwhile we have people bitching and moaning and meanwhile, as a Firefox user, I go to Safari when I encounter a website that doesn't work in Firefox. Because at the end of the day, developers are forced to test in Safari, and that is a Good Thing. No developer is forced to test in Firefox.