Browser monoculture is bad for the open web and if all we have is Webkit (Safari on iOS, Macs) and its fork Blink for all the Chromium browsers, then the web will start becoming a mess of proprietary extensions instead of open standards.

I see this claim often. As someone who learned web dev during the days of IE dominance, I don't understand it.

Internet Explorer never kept up, especially after IE6 reigned supreme. They weren't "a little behind" or didn't have some more niche APIs missing or implemented in a buggy or proprietary way. It actively ignored standards, it didn't receive real updates for a long time (IE11 being the fruition of what the best they could offer was) and generally with few exceptions (namely, the invention of CSS Grid and XMLHttpRequest) generally degraded the ecosystem for over a decade. It actively held back companies from adopting new web standards. Its why polyfilling became as proliferated as it is now.

Safari / WebKit has not induced any of this. Yes, sometimes Safari lags behind in ways that are frustrating. Yes, sometimes Apple refuses to implement an entire API for political rather than technical reasons (see the FileSystem API), but largely it has managed to stay up to date with standards in a reasonable time frame.

While their missing or subset implemented APIs can feel really frustrating, they haven't actively held back any work nor the mass adoption of newer browser APIs.

Apple has their faults, but this isn't even close to the drudgery that was the IE heyday era.