That's a fallacy, as the "OS" portion of Windows existed long before the MAC and was copied from Digital Research's CP/M. Now the GUI portion, which I believe you refer to as the MAC OS was copied the Xerox Star so if Microsoft stole from anyone it was Xerox.
With this much ad revenue at stake, I think someone will rise, and better yet, it will be a disruptive technology that will possibly take Google by surprise as they ride into the sunset milking AdWords/AdSense for all it's worth.
Don't forget, when I started in computers we assumed IBM would dominate and that continued with the first IBM-PC as IBM thought computers were the power, not the software, and Microsoft and PC-Clones blind-sided them and POOF! the household word of IBM became second to Microsoft.
A few years back the newspapers, magazines, radio and TV thought they all had a lock on their advertising and along comes this thing called the internet and Google and POOF! their profits went into the toilet, subscriptions plunged, advertisers flocked to this new medium.
So the same thing can happen to Google, we just don't know what it is at the moment, but it will happen.