> Would it cost more than a billion dollars to assemble an elite team of a few hundred people to build a new OS? Would Apple, Microsoft, or Google notice the billion dollar cost? Or even two billion?

It's not a matter of money, resources, or talent. For reference: The Mythical Man-Month, waterfall process, Windows Vista.

Building something of this scale and complexity from scratch will take a lot longer than even the most conservative estimate and will take many more years to shake out the bugs. Again, remember Windows Vista? And that was not nearly as revolutionary as the things you suggest.