No I expect them to support it until it is no longer feasible to support it. Rather than developing a new OS to develop it, fix the one that's running. But in a consumer based economy, they always want you buying new instead of fixing old. Many things we have are only not usable simply because they are made to be obsolete, rather than being made to last.

Also, expecting something to last until it isn't actually viable anymore (not just because they do everything they can to make it non-viable to force you to move to the newer model) isn't the same as expecting it to be supported forever. Grow up with the extreme examples and realize that just because I realize things should have a longer end of doesn't mean I want it to last forever.

I do not remember MicroSoft ever saying that they cannot port DirectX 12 to Windows 7, but I bet MicroSoft can port DirectX 11, DirectX 11.1, DirectX 11.2, DirectX 11.2, and DirectX 12 to Windows XP.

There is already a way you can force DirectX 10 to Windows XP.

I bet MicroSoft can port DirectX 11.1, DirectX 11.2, DirectX 11.3, and DirectX 12 to Windows Vista, as well as port DirectX 11.3 to Windows 7.

Even more so MicroSoft can port DirectX 12 to Windows 8 and Windows 8.1.