Ahhhh.. the thing is, insurgency/guerrilla warfare is almost certainly not going to end once we pull out of Iraq and Afghanistan. In fact, to a large degree, it is probably going to be one of the dominant modes of warfare in the 21st century. It's not like Iraq/Afghanistan are minor detours and we'll get back to fighting old-fashioned conventional wars afterwards. Iraq and Afghanistan show us what a lot of conflict is going to be like in the coming decades: Asymmetrical, non-linear, low-intensity, and fought against an unconventional or irregular enemy.
EDIT: Don't get me wrong, I agree that it's important to preserve a strong conventional capability. Just because unconventional conflicts are going to be common in the coming decades doesn't mean conventional wars are a thing of the past. But that conventional capability has to be balanced with unconventional capabilities; we have to be able to do both well. I think the idea behind Gates' proposal is that our current defense budget is weighted too heavily towards conventional capabilities, to the detriment of our efforts in current and future unconventional conflicts, and he wants to move it more towards a balanced mid-point.
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