IMHO "emulating" the (fairly small) subset of the traditional Win32 and DirectX APIs required for typical games might even make sense on Windows itself, should Microsoft one day decide to deprecate Win32 in favour of something more "modern" (heavy air-quotes).

The window creation and input event handling of Win32, together with D3D9 and D3D11 (the really popular versions of D3D), and whatever is the current audio API on Windows (this seems to be the least stable area) have proven to be quite fantastic for games, maybe the best set of low-level APIs needed by games outside of actual gaming console.

Porting those APIs to other platforms, instead of porting the games doesn't seem such a bad idea at all in hindsight.