Ethanol from corn is unlikely to become a major source of fuel in the US. To produce as much ethanol from sugars made from corn as have the energy content of 4 gallons of oil you need to invest about 3 gallons of oil (for tractors, fertizer, distilleries, etc.).
Therefore, even if the US had unlimited farmland (which it doesn’t), if all cars in the US were to run on 100% ethanol the US would still have to import oil for farming corn that would be the equivalent of 75% of what cars use today. It’s one of the worst ideas anyone has come up with to reduce fuel imports.
By contrast, ethanol made from sugar cane in tropical climates such as Brazil yield 8 gallons for every gallon invested. The US would be far better off importing more cane alcohol from Brazil while keeping up research on alcohol from cellulose (such as straw) than to pamper the domestic farm lobby.
Cars that are inefficient will price themselves off the road. If a car costs $xxx per month more to run than a more efficient new car, then at a certain age it will get scrapped and melted down for steel, just like all cars eventually are.