Critics like the ETA point out that a great deal of energy is used in the creation of automobiles, so it's advising people to hold on to their current rides for as long as possible. According to ETA Director Andrew Davis, "Altering the way you drive and keeping a car longer can be a greener option than buying new. Even if the new model you buy is more economical, once you take into account the energy needed to scrap the old car and build an entirely new one the overall benefits are likely to be tiny."
You could argue that this line of reasoning overlooks the fact that global auto sales are extremely low compared to the levels of the last few years, which is leading to a steadily aging fleet of vehicles. This being the case, there are sure to be a number of older vehicles on the roadways that will need replacement within the next few years anyway. Of course, whether or not the government should subsidize their replacement is another issue entirely.
[Source: Channel 4]