I don't think classic cars will ever be "banned". It simply doesn't make sense for the relatively small numbers of cars that would qualify. What would be the overall benefit? To do so would be a politically unpopular stance, albeit with a small fraction of the voting public.
There are plenty of vehicles that eat a lot of gas. Although pressure is brought on manufacturers to raise their CAFE numbers, BMW has (I read elsewhere) always paid the most CAFE standard fines of any manufacturer. I don't see that changing until/unless BMW goes all-electric or at least, all hybrid. Electric and hybrid vehicles are only less than 4% of new cars sold.
My wife and I ponder a plug-in hybrid occasionally. There are at least five Priuses in our immediate neighborhood, and their owners love them. Still, why would I spend $20K-plus to replace a car I already have? The payback period is ludicrous. Let's say we drive 10,000 miles a year at 20 gpm, that's 500 gallons of gas at $3/gallon, or $1500/year. It'll take 13.3 years to recover if my hybrid is fueled for free. The new car would come with higher insurance costs too.... gasoline still rules the economics, except to politicians, or unless you have to buy another can anyway. Then there's the used market waiting for you. Sorry for going on but I are engineer and Total Cost of Ownership is very much in my mind of late.
Still, stranger things have happened, and continue to occur.