[...] as long as MS does a good job keeping this abstraction layer stable OS upgrades don't concern you much. The problem is that you have to keep them stable

[...] as long as MS does a good job keeping this abstraction layer stable OS upgrades don't concern you much. The problem is that you have to keep them stable to a much higher degree than you might think at first. Obviously you have to maintain the functionality as described in the API documentation but that is far from enough. If the implementation has a bug, you can not simply fix it because there might be software either relying on the wrong behavior or having implemented workarounds that you might break with the fix. And no matter how often the API documentation states that this or that is not supported or undefined behavior there will be software doing it anyway or relying on what ever undefined behavior meant in the version they developed against. You return a list of something and say nothing about the ordering in the documentation but as an implementation detail it happens to be in some specific order, if you wait just long enough there will be a program critically depending on the ordering you choose. This is an endless nightmare especially if your APIs have as many different consumers as the Windows APIs.