In the 60's and 70's one could actually work on a car.
IMO today's cars are almost impossible for even the above average amature auto mechanic. The special tools and computer diagnostics that are required, make it prohibtivly expensive to modify ones car. A lot of dealerships rely on calling 'factor tech support' for a problem diagnosis.
Today's muscle car is not just a big block, but rather an extremely expensive, sophiscated, turbo or supercharged engine.
Young people can't afford to own / maintain this kind of car.
Several of my divorced friends (over 60) have replaced their wives with sports cars
But yeah, I think the concept will be dead in 20 years. You can get stock cars with respectable performance. Besides, there's nowhere to drive them!