Achoo22,
Support->
Okay, fair enough. Extended support for major security etc is 2020. There will be other code as part of "mainstream support" that won't exist so I'm not sure how that will affect things.
Backwards compatibility->
Sure, Microsoft could easily add support for W7 with the wireless XBOX ONE receiver. Maybe they will even do that. However, the reason which is likely obvious to most is that Microsoft is aggressively pushing people to move to Windows 10.
The big deal with Windows 10 is cross-device integration (console, laptop, phone, PC). So, this is a rather unique situation. Microsoft wants some integration between Windows 10 gaming and XBOX ONE gaming so has decided to push the controller exclusively to the XBOX ONE.
(Let's be honest, it's about forcing people to Windows 10 as making the wireless driver work on Windows 7 would be childs play and likely will be hacked anyway)
*As for the "45%" on Steam, keep in mind that's today. That will be quite different in a year.
**My "issue" I guess with your statement about it being "insane" is that I think you're implying that Microsoft doesn't know what they're doing because a large number of people are on Windows 7/8 still. I don't agree. Microsoft has a plan, and it's actually pretty good (cross-device integration, including integration between W10 gaming and XBOX ONE). Pushing people towards that benefits them far more than any sales of XBOX ONE controllers. Besides, as I think I said you can still just buy the XBOX 360 controller anyway and it's fine.