Another answer to this question is from the perspective of professional software management. Specifically, the cost to Microsoft of obsoleting the Win32 api is too great. That is, if they were to NOT support it in Windows 9, for example, then every customer who is currently running a program that requires the API would re-evaluate their need to use any Microsoft products.
Some customers, i.e. more than a few, would say that Microsoft products are too expensive and would take the opportunity to either 1) remain on Windows 8 and prior until the end of time and NEVER buy another product from Microsoft; or 2) convert their business systems to using a *nix or an Apple system.
Note, the above is NOT speculation on my part. This very thing happened to IBM in the 1960's when they forced their customer base to migrate to the 360 series of hardware. At the time the 360's were completely unlike the previous IBM mainframes and customers had to significantly re-work their existing software in order to run on the new machines. Many IBM customers choose to go with Sperry-Rand and other hardware vendors where the cost of a new machine and a conversion was less than the cost of an IBM 360 and a conversion.
IBM almost went out of business because of a businesses right to freedom of choice! They (IBM) learned a lesson. To this day it is still possible to run programs written in the 1960's on a present day IBM mainframe.
