Apple don't have to support much hardware. They have a rolling 5 year support pattern, release new hardware at the most yearly and release a new OS every year. That means if they released new Macs in every single configuration every single year and you discounted RAM, CPU (aside from SMP) and screen size (apart from Retina which may need different drivers) differences and they would have to support a total of 12 versions of 5 new Macs a year, if you want to be generous:
(I counted the following from the current generation.The only thing I think is missing is the optional Graphics Cards for the Mac Pro: Mac Pro Quad, Mac Pro 8/12 core, 27in iMac 675MX, 27in iMac 660M, 21in iMac 650M, 21in iMac 640M, Mac Mini (All configs use the same driver set), MBA (All configs use the same driver set) ,13in MBP, 13in MBP Retina, 15in MBP, 15in MBP Retina)
Thats 60 Macs max. Mountain Lion currently supports around 30 (I count 29 but many of them could be revisions). Nothing compared to the average roster of, say Dell, especially considering that combining it with Microsoft still wouldn't net you a company with the financial clout or net worth of Apple.
And about the 'heavy handed' code bloat, where even Windows ends up looking good in comparison. At least when Vista came out MS listened and made 7 faster. 8 is even faster believe it or not, making it run faster than XP did back in 2003. That sounds like the OS Apple would make in my wet dreams, and if MS can do it, I honestly think that Apple is not doing it deliberately. Only one reason I can think of; Planned Obsolescence.
EDIT: Also, the post above makes an important point about Digi and how it always used to hold us up on updating due to compatibility. That point is that if there was an incompatibility, or things were buggy, Digi told us, and often had version checks in the program that refused to run on the suspect newer versions of the code until updates were released. Still no word from Apple...