The only feasible way to have cars and trucks in a zero-net-emission economy is to make them electric. Unless you want to scrap all cars and trucks, the electric versions must eventually be developed and mass-produced.
Used car prices are practically always in freefall. You can expect a car to depreciate by over 60% in a decade, even more if it's driven often. I don't expect this to radically change. As demand for gassers lowers, so will the rate of new gassers being purchased, and equilibrium will be kept. As for the really old gas cars on the road- those will maintain their value because we are a long way away from electric cars being competitive on a sub $5000 price point.
Freefall is a metaphor here. The way you define it, maybe it's already happening, fine. Let's grant you that the fact that freefall happens is a given. The rate of freefall can still change, drastically.
In California you can lease a Leaf for $95 / month after subsidies. That actually is pretty competitive with a sub $5000 price point on a competing ICE car that would require the purchase of gasoline.