But are there any statistics for the carbon cost of trashing and replacing still-working vehicles (whose manufacture-related emissions could be considered a "sunk cost") with new ones, relative to the carbon saved by the more efficient engines?

I'm not much in favour of bailing people out of foreseeably stupid and irresponsible decisions like buying an SUV for private use, unless the benefits to the whole of society are both clear and substantial.

Are they?