There are currently 2457 (more or less, depending on region) games in the Windows Store which aren't available pre-Windows 8/RT.
I don't see what volumes that speaks about it?
And what studies are you basing that on? Backwards compatibility with 2 or 3 previous gen operating systems has always been a concern of developers. As a matter of fact many games released today are still compatible with XP SP3. Besides, the game(singular, not plural) you linked to will also be released on windows smartphones, which speaks volumes about it....
I don't see what volumes that speaks about it?