Your "dream" there fuels some of my nightmares. You may discount the potentially devasting threat of a possible massive ecological collapse from relying on one single implementation of web rendering because you don't think open source projects can die because "someone has the resources to potentially fork it", but open source is no guarantee.
Beyond that, even discounting the threat of ecological collapse due to a single codebase, most of the web's best standards have come from lessons learned from multiple implementations. Without multiple implementations we lose a lot of guide rails away from bad single vendor "standards". Even if you think downstream players like Microsoft can act as a big enough check on Google's rubber stamp in standards boards like WHATWG, there's a huge difference between Microsoft developers evaluating potential standards because they have to build their own implementation of them versus just reading them in code review to accept an upstream PR they didn't need to write themselves.
Also, as close to a single implementation as WebKit and Blink still sometimes are, it's hard to argue that Apple isn't already still a contributor to the WebKit/Blink/Chromium Hegemony. It doesn't matter if iOS opens up to other browsers or not right now while WebKit and Blink are still so close to mostly not matter. Apple is sort of protecting everyone from a true V8 (JS Engine) monopoly, but the renderer has always been close enough to not matter to those terrible web developers only testing websites in Chrome and assuming they work "everywhere" and supporting a "Works Only in Chrome" agenda.
Beyond that, even discounting the threat of ecological collapse due to a single codebase, most of the web's best standards have come from lessons learned from multiple implementations. Without multiple implementations we lose a lot of guide rails away from bad single vendor "standards". Even if you think downstream players like Microsoft can act as a big enough check on Google's rubber stamp in standards boards like WHATWG, there's a huge difference between Microsoft developers evaluating potential standards because they have to build their own implementation of them versus just reading them in code review to accept an upstream PR they didn't need to write themselves.
Also, as close to a single implementation as WebKit and Blink still sometimes are, it's hard to argue that Apple isn't already still a contributor to the WebKit/Blink/Chromium Hegemony. It doesn't matter if iOS opens up to other browsers or not right now while WebKit and Blink are still so close to mostly not matter. Apple is sort of protecting everyone from a true V8 (JS Engine) monopoly, but the renderer has always been close enough to not matter to those terrible web developers only testing websites in Chrome and assuming they work "everywhere" and supporting a "Works Only in Chrome" agenda.