All you say here is true, but wouldn't we save some if they were all light fighters? Also-believe it or not-we have a lot of tracks mothballed. An instructor I had at Camp Shelby said that there were more M1s stored there than at any other post in the world (including Hood). They have them stored in rows of warehouses and they don't even have the resources to maintain them. Those 18 divisions of 1991 were fully equipped and those tracks (outdated though they may be) are in storage somewhere-at least some of them.
Technically, yes, and I agree with your premise. Here's where that ship has run aground, though.
The idea behind the Army transformation is to create a light, mobile, lethal force. One that will tear apart other conventional military forces fast, accurately, and with minimal casualties. Even in it's semi-transformed state, that's what we saw in 2003 against Saddam's army. The Pentagon wants a force that can do that against anyone. Like, say, China.
Having a large mass of light fighters, against their large mass of light fighters, runs the risk of high casualties. That's why the brass, increasingly gunshy of the media, is averse to the idea.
What we're starting to see is that the enemy doesn't have to lie down and play dead, just because Force XXI shot all of it's tanks from 2 miles away. Our light, lethal, agile force is death incarnate against enemy armor, but it isn't heavy enough to occupy and enforce our will on a resisting populace. That takes boots, and lots of them.
So, cost savings aside, we need more ground pounders, especially of the SOF variety. Not peacekeepers, not warfighters, but surgical instruments of U.S. power. People who can actually accomplish United States foreign policy objectives once major combat operations stop, and the tough work of winning the peace starts.
We never want to lose the 'death incarnate' conventional power. It's the best thing our tax dollars go to, hands down. We just need to add useful components, like the light fighters you mention, to round out our capabilities. Spending three years losing what you won in three weeks is a hard thing to do. It didn't have to be this way.