What kind of gamer is still using Windows 7 anyway? I could see using it for businesses and such, but since you can get extra performance from Windows 8.1, and gamers need every last bit of performance, Windows 7 isn't the best choice for gaming anyway.
Really, Microsoft deserves the black eye for their high-handed approach to making everyone use a tablet OS on a desktop with Windows 8, but Windows 8.1 is just better than Windows 7. I have both, and can dual boot, and I never go to Windows 7. It's obsolete, and crude by comparison, after you set up Windows 8.1 so it works how you want. But, thank goodness for the uproar, or we'd still be booting into that idiotic metro screen.
Even so, forget DX12 for Windows 7. The OS is completely obsolete and bloated. They've already said they'll be supporting Windows 8.1, although I'd prefer they just forget that because Windows 10 will be a free upgrade, and will fix the lingering interface issues with Windows 8.1 (the idiotic charms, and the right click start button), while presumably improving performance even more. Why bother supporting either?
Of course, Mantle will support all of the OS varieties, so it always an option on the people who cling to the decadent Windows 7 OS.