The problem is that WebKit isn't really a competitor to Blink:
1) It holds this marketshare mostly because of the forced monopoly on iOS. It's not technologically particularly far ahead or even has a browser implementing it with good marketing.
2) It exists on platforms other than macOS and iOS, but it is being optimized specifically for those platforms and only there can kind of compete with the other browsers. So, if you're not on macOS/iOS, it is hardly a competitor that you could choose from.
3) Chrome's Blink engine was forked from WebKit, so they are actually very similar in a lot of ways. A Blink-WebKit-duopoly would still have many of the disadvantages of a monopoly, like security vulnerabilities being shared and certain innovations being harder, because they have the same architecture.