That's how 90% of software licensing works...
Subscription aside
Whatever the reason DX12 won't have massive uptake for a while if Windows 7 does not implement part of it,just as DX10 did not since it was tied to Vista.
Even DX11 was available on Vista too,meaning it had a decent marketshare when compared with the initial Windows 7 sales numbers,and look how long it is has taken for DX11 to become common??
DX11.2 is barely a blip on the radar since it is a Windows 8 exclusive.
On top of this many games are still DX9,like Skyrim for example and a lot of games only implement some DX11 features to a limited degree.
Windows 7 only got such decent sales at the beginning since it was dirt cheap(under £35) and even Windows 8 adoption(despite it not being as sucessful as MS wanted) was again down to it being cheap too at launch.
Windows 9 not only has to be a big upgrade over 7,but it needs to be well priced. The problem is if instead of a £50 to £70 OEM license,it becomes £40 a year license,then it fast gets expensive soon,and people will just stick to Windows 7.
MS already makes the most money of all software companies,and this is just another case of a company ripping off gamers,since no one apart from gamers really needs to be upgrading to the latest version of DX12.
Its bad enough with overpriced ROG and Fatal1ty branded meh,it seems PC gamers are getting increasing taken for being mugs,while at the same time consoles are the lead platforms for more and more games.
However,I expect XBox One owners need not worry - MS will just apply a free update!
Even though I am not enthusiast ATM at the state of Linux gaming,if Windows 9 turns out too expensive longterm,then it would be another push towards improving Linux gaming.