> would Google have ever emerged as a dominant player if Microsoft were free to compete with them by restricting access to their website at the operating system level?

Yes, the scenario you seem to be imagining is logically impossible.

Microsoft can't just block traffic to random URLs before they emerge as a dominant player somewhere. The course of action only makes any sense if Google has already achieved dominance.

Could Microsoft shake a website's existing dominance by blocking web traffic to it at the OS level? Of course not.