So then what you are saying is that net/net it will have very little environmental impact being that such a small % of cars (within the big picture) are being traded in. So if the motives aren't "green" then what is the gov'ts motive? To stimulate the auto industry? Why does the gov't get to pick which industry to stimulate? Give me $4500 towards a new boat and then we will be talking!!!
Just speaking theoretically here....but let's say you just invested $40Billion in a business or two. Somebody says throw another Billion out there as a rebate, and maybe you'll start seeing business pick up across the board. Ya' know, help "stimulate" some return on your original investment.
So that "could" be the gov't's motive, and it certainly justifies which industry they choose to support.