I though the point was having a vertical market on your product not pushing innovation.
I can see that either method would push innovation, except you couldn’t have very many CPU and GPU producers, as those are too expensive. Of course in cars companies share each others’ engines too.
If Microsoft would start following Apple's example, and start building their own computers, designed so that they would only run Windows, and writing Windows so that it would only run on those computers and a specific set of peripherals what happens to all the other PC and MB manufacturers?
About the only market they'll have is linux.