Truth is, without going into alot of details, future classic cars are simply what the current generation really likes/has, or really wants but can't afford.
Thats the reason of the inflated prices of cars from the muscle car era. The baby boomer generation is now middle aged, and are trying to buy back the car they had in their youth, or the car they've always wanted in their youth.
For the most part, those cars came (very) short on just about every aspect compared to new cars. Today we are left with vivid images from movies like fast & furious where everyone was driving around in a modded 12s race-spec car. Thats not the case, and many would not like driving one of those older cars around as a daily driver.... non-hydraulic clutch, drum brakes, minimal creature comforts that we've come to expect on the most basic cars today (power windows, delay wipers, cruise control) etc...
Which is why I think we are going to see a rapid decline in 60's/70's era muscle car values once the baby boomers pass on. The "silent generation" (baby boomers parents) are the ones that bring the 30's and 40's era cars to car shows, not baby boomers. Same goes with the muscle cars.
Of course rare, limited and low production cars will always be collectible regardless of generation, but for the run of the mill mass produced car? How many fully restored Model-T's (not hot rods) do you see at car shows? How many are owned, due to want (and not passed down) by baby boomers or younger? How many baby boomers seek out 1920's-1930's era cars? A far less amount then the number that seek out cars from the muscle era. Same can be said to compare my generation to the previous. Today generation X and Y like the muscle cars because of the "bad azz" appeal and big engines, so you do see a (slight) varied ownership age at shows, which again, has alot in part due to the various movies that bring back the classics to the scene. For the most part, my generation is into tuner/import/sports type cars and not big lumbering muscle cars, and that's going to be the type of cars that will be popular when my generation is middle aged. People will be saying "my best friend has one of those WRX STi's back in college" etc... These are going to be the types of cars that will be deseriable "future classics" because when my generation is middle aged, they will want to relive their youth as well.