When Chrysler introduced 4-wheel hydraulic brakes on lower-priced cars, the mfrs that did not have this technology predicted gloom and doom because those Chryslers “would stop too fast”.
When seatbelt use was mandated in many states, some folks refused to use them because “thousands of people will be trapped in burning cars when the belts won’t release”.
When airbags were introduced, some folks predicted that more folks would be killed by airbags than would be saved by them.
When automatic transmissions were introduced, some folks actually predicted that drivers’ left legs would begin to atrophy from disuse.
When catalytic converters were introduced, some folks intentionally destroyed them by fueling their car with leaded gas, because…well, just because cat converters were going to shake up their automotive world in a way that was alien to them–and they found this change to be threatening to them.
Just as our republic did not cease to exist as we knew it when women were given the vote, or when the military and the schools were desegregated–despite predictions to the contrary from the Cassandras of those times–cars as we know them will not cease to exist simply because of new fuel economy regs.
What will happen is that technology will advance more rapidly, simply because there will be an increased push for new technology in this area, and this new technology will produce most of the necessary increase in fuel economy.