No it won’t be the end of muscle cars, as they are a small volume compared to the high volume cars, what it will hopefully mean is the end of giant pickups and SUV’s used to commute in. I’m all for the return of small fun cars.
And cars designed to run exclusively ethanol will NOT get worse mileage, they will get better mileage and have more power because they can run higher compression than gas engines. It’s only when you run ethanol in engines designed for gas that you get worse mileage.