The idea of Windows as an operating system is purely a product of the Microsoft marketing department, and not some law of nature. When Windows 1.0 appeared, it was a separate program you loaded on top of MS-DOS. Same for Windows 2.0 and 3.0 up through MS-DOS 6.22, the last standalone version of DOS sold by Microsoft. That way, they thought at the time, if Windows ever proved to be a commercial failure (this was far from certain and it is easy to claim Windows WAS a failure before 3.0), it would have been easy for Microsoft to punt back to MS-DOS.