Trouble is not only the complexity of a web browser (and security thereof) but that Google's "living standard" is a constantly shifting target. Even a company like Microsoft couldn't keep up with it and gave up. So long as Google is in charge and has infinite* resources to throw at it, nobody else is ever going to be able to keep pace.
Apple works around that by not really trying to keep up. It implements features in its own time (or else some third party contributes code to WebKit). But iOS gives Apple a captive audience which means it has much more freedom to be "slow" compared to Google. As much as I might philosophically dislike their policy of only allowing their own web engine on iOS, it is the only thing keeping WebKit alive as a major alternative.
Apple and Microsoft both have resources to dedicate as much man/machine-power as Google does for their browsers if they wanted to. They just don't want to play the neck-and-neck game any more. MS has opted to simply use Chromium while Apple as you say goes at its own pace. But they do have the capacity to match Google if they choose to, given some ramp-up time.
Apple works around that by not really trying to keep up. It implements features in its own time (or else some third party contributes code to WebKit). But iOS gives Apple a captive audience which means it has much more freedom to be "slow" compared to Google. As much as I might philosophically dislike their policy of only allowing their own web engine on iOS, it is the only thing keeping WebKit alive as a major alternative.