What the CARS program has done is brought forward new vehicle purchases--purchase that might have been made next month or next year are made today. For the immediate term, that is a benefit.
Does it address issues of demand tomorrow? No. Absent other forces raising demand, the CARS program stimulates demand for new autos today at the expense of a decline in demand after the program has run its course. A program like CARS does not produce sustainable demand on its own.
A tax credit, while slower to take effect, would have been a more longer-lasting stimulus, as it would have been a permanent downward shift in the nominal cost of a new vehicle to the tax payer.
The trade off is a larger but less sustainable stimulus today vs a slower but more sustainable stimulus tomorrow.