Expect Microsoft to screw over their PAYING customers, again... like with XP and DX10... which was by choice. When a hacker can make Windows98 run on an iPad, I think a multi-billion dollar company can make their OWN API work on anything.
So, to *PUSH* Windows 7 users into Win10, they will NOT release DX10 for Win7... which is going to have the same effect for XP/DX10... Not many people will give a <Mod Edit>... they'll stay with Win7. Sure some games may "require" DX12, but will only use DX10 - maybe DX11 effects... and with MS not supporting gaming for PC... it goes back to... "Who cares?"
The Game developers won't care because they have a much better platform to develop on with the PS4. Remember, a console game that sells lousy - would be a big seller for PC market.
For business users... (the primary user base for Windows, since it's main function is to run MS-Office), they could care less about DX 9, 10 , 12 whatever. So again, MS will burn all their gamers and slow down the adoption rate for DX12 in the hopes of getting a few thousand converts.
Only way to get people to REALLY want Windows10, is if there is some sort of holographic technology that can't work with anything thing else... but that is hardware limitation, not an OS.