The real ,proble with a program like this is that it doesn't really seem to know what it's supposed to do.
If it's to promote new car sales the effects are laughable: offering peanuts for old "clunkers" isn't going to shift people from them into mcu more expensive new cars. At the very, very best it simply puts a floor under to lowest price for an old clunker and that might, if they're very lucky help increase used car prices somewhat (though the effect will decrease as the value of the used car increases- a $1000 for a clunker will affect used cars in that price range but have a decreasing effect for used cars costing $10K, or 15K or more). I really cannot see this as having any real effect on new car sales.
If the policy is intended to take older, less efficient and polluting cars off the road it seems misguided in simply equating old or cheap with inefficient. A far better and more effective approach would be to institute emissions limits and make passing them a condition when inspecting cars. It'd be cheaper, and older classics should have an exemption- a grandfather clause. It's not the 30+ year old classics or old even the few bangers still banging about that are the problem- it's the masses of newer cars that are more of the source of these problems.
I guess this is political... but the thinking is muddled, and the policy ineffectual.