I don't think speed shops will ever close. There is a big bunch of 5.0 mustangs in the sacramento area, and most of them don't know crap about them so they pay some of the speed shops to tune them up for them.
I think its pretty much like the music or entertainment industry. Sure there will always be the classics, but more come out every year that new people fall in love with and go out and buy. So i think that fewer people will be fixing up 50, and 60s cars but more 70s and 80s cars. I know that i can relate to late model muscle cars, so i definately know i'll be building some in the future.