"…because it's already happened and the world hasn't exploded."
It's hard to know now what the very–long-term effects will be but John's point that the web has benefited from Chrome's creation is hard to argue with. As for whether it's benefited more or less than if Chrome had used a different rendering engine, we'll never know…
That's an interesting assumption to make: Just because Safari or Chrome hypothetically used Gecko doesn't mean that competition would've halted. Why couldn't it have been any different from when Chrome used WebKit as its basis and absolutely trounced Safari?
I think we should at least touch on the real reason Chrome is the de facto standard and baseline for web page rendering: it's essentially to front end development what IntelliJ is to backend.
It's hard to know now what the very–long-term effects will be but John's point that the web has benefited from Chrome's creation is hard to argue with. As for whether it's benefited more or less than if Chrome had used a different rendering engine, we'll never know…