You have to think ahead. AMS development isn't finished yet - its way behind the original plan. This would suggest Reiza17/AMS2 is perhaps 1.5- 2 years away. How many people will still be using ageing Win7 PCs in 2 years time? Ideally you would offer both DX11 and DX12 options.
I was one of them until January this year. But Kunos, ISI, SMS and S3S had all dropped support for XP years before, even though some of them were/are still using DX9 graphics engines. If you run an old PC you can't expect to run the latest games.
Win10 isn't so bad. Yeah it has some default features that spy on you, but not like that stuff is hidden or forced? Just disable it Games that offer support for both DX11 and DX12, mostly run better with DX12. Most gamers I know have updated to Win10 and haven't really heard many complaints. Vulkan or DX12 is the way to go. DX11 in the bare minimum
I think the latest ransomware catastrophy shows that sooner, or later everyone will have to either learn to love Windows 10 or start learning Linux. Dropping support for 7 and 8 is all Microsoft has to do and they would probably do it already if not for the governments and business. As for DX12, so far pretty much any game I have tried on my 980 runs slower on 12 than on 11 and Vulkan beats beats the crap out of both. Also, DX12 does not seem to have any graphical improvements over 11, no new shaders, nothing.
FWIW, MS provided an emergency patch for all those 'not supported anymore' OS. People don't stop using software because the vendor drops support. So it would solve absolutely nothing.
Where are the windows 98 users? MS provided an emergency patch because of the lazy people who refuse to upgrade infrastructure at a hospital for example.
Also, a lot of computers just did not run well when windows 10 was forced down their throat. It happened to me twice. That annoying windows 10 download notification in my task bar, made me assume it would run on my computer. Two times it ended badly. I am running 10 now though, on a third machine. It's ok, lightweight enough, and means I can access more modern games. But if windows 7 was more lightweight, and had more modern gaming options, I'd be pretty tempted to go back to it.
Notification on the task bar isn't such a big deal, the problem is that extremely annoying highly intrusive blue full screen notification that tabs me out of full screen games and often causes programs to crash.
I'd love to participate in the next game. Any indication if Reiza would open a forum section and if it would involve beta players like we have with the Reiza51 forum? It's 2017 after all and I don't seem to read much news about the next installment lately .