Security patches, while not trivial or cheap, are vastly simpler than either the backward compatibility problems I described or the issue at hand with incompatible video drivers.
You can't just add programmers to mate cutting edge OS kernel features (ie leveraging recent videocards) with old hardware. None of us can speak authoritatively on the case in point, but it seems painfully optimistic to think you can pull this off without introducing a lot of complexity. And then, what happens if the 10.8 team tweaks the kernel during a point update? Yep, better double or triple the testing team.
"Oh, it looks like it'll take some work to get 10.8 working with all of our old AMD chipsets - either we delay release until fall (after Windows 8, unfortunately) or we put some of our "A" engineers on it."
Complexity is the keyword. Complexity and mediocrity go hand in hand. Backward compatibility is a noble goal in many ways, but every generation back of software/hardware you try to maintain exponentially increases the complexity you deal with instead of putting a better new product in user's hands.