Maybe, but you're making a bunch of assumptions here - people might well replace an old clunker with a small compact for only $3000, but you can't take that for granted. People who don't own a car already might enter the market at this price (adding to pollution and congestion) or those who have efficient cars might replace them with an equally efficient but much cheaper car and and simply pocket the savings, for no marginal gain.
I agree the existing system isn't very economically efficient but ISTM that you're assuming rather than demonstrating the efficiencies that would result from your alternative scheme.
My example was really meant as a thought experiment rather than a credible program, but obviously governments and institutions can formulate such plans with requirements for the replaced vehicle, e.g.
I agree the existing system isn't very economically efficient but ISTM that you're assuming rather than demonstrating the efficiencies that would result from your alternative scheme.