I see several problems with this. I also love the software over the hardware. But I see them complementing each other. If OSX is decoupled, it does sell a lot of copies, however the amount of overhead that goes into supporting lots of different software configurations (not to mention legacy code) increases dramatically (just ask Redmond).
This would create a staggering performance problem for OSX and level the field in that regard. I find it amazing that my iMac is 3 1/2 years old and is not only ABLE to run Tiger, but it is FASTER than when I had Jaguar on it. This is something Microsoft cannot do. Linux has been able to do some of it. Linux faces the driver problem as well. Once you support a large range of hardware, you have to work that much harder.
Now I know that Apple could limit the specs that OSX would run on and say if you use anything else it would not be supported. That won't fly. I'm not saying that Apple won't get out of the computer harware business, I'm just saying that I don't think it's as much of a given as you think.
Five Reasons Why There Will Be No Macs in 2010