M$ is pretty much killing itself in the consumer market and is rapidly reaching the point of no return and perhaps even crossed over.
Windows (all versions): 85% and stable
OS X: 11%
Misc (possibly mis-ID as desktop): 2.5%
Not sure where you got those figures. Linux Desktop Market share is now at 2.27%. Not huge but definitely increasing.
One third of the 85% above is now using Win10. Half the gamers on Steam now run Win10. With Ryzen and Kaby Lake there is no Win7 support. Sorry to disappoint you, but even as people are holding on to Win7 there zero evidence of any migration away. When push comes to shove I imagine most will begrudgingly upgrade like they did with WinXP.
Again I will refer you to the URL. Windows 10 is approx 25.3% with Windows 7 approximately 47.2% and surprisingly Windows XP at 9.17%. Even Windows 8.1 is at 6.9% so that tells you how popular Windows 10 is, although as people throw away their old windows machines and purchase new ones then Windows 10 market share will increase.
In the motherboard BIOS there is an option for "Other OS" and I initially installed Fedora 24 (now 25) on the Z170 (takes Sky Lake) without any problems so I don't forsee any issues with the motherboards for Ryzen (when it comes out) or Kaby Lake which has the same LGA 1151 socket as Sky lake and will run on Z170, H170, B150 and H110 series motherboards . It will be possible to install Windows 7 (if you can get a legitimate version or do you pirate it?) under the Other OS feature but like you have said it will not be supported by Microsoft.
As far as PC games go, Microsoft Windows dominates although if you go to Steam and look at the number of games available for Linux and SteamOS there are over 5,000 and some are AAA. Good luck finding the time to play them all.
The majority of people will not upgrade to Windows 10 unless Microsoft use the same tactics when they made the OS a free upgrade if you had a legitimate copy of Windows 7 or Widows 8.1. If you wish to upgrade now you have to pay for Windows 10 and most people will not do that unless they replace their PC which in the majority of cases the new PC will come with Windows 10 as the default OS.