The only worry here, as always, is that an implementation will come to define the standard rather than the other way around.

Note that coding to an implementation rather than a standard does stifle innovation, because when you code to target the implementation you inevitably rely on bugs in the implementation, without ever knowing or trying to. From the perspective of an implementor, would you fix a bug even if it meant breaking an unknowably large number of websites? If not, you've adopted that bug for the rest of time, and every subsequent implementation, ever, will be forced to implement the bug as well when they could be busy implementing new features. Now compound this by thousands of bugs, and reflect on all the time spent implementing Quirks Mode in every browser engine that exists today.

I do agree with you that in a perfect world we wouldn't need multiple implementations, because bugs would not exist and the lone implementation would track the standard perfectly, and because the standard would be controlled by neutral third-parties. But I think it's much too soon to declare the current era the browser endgame.