How many CURRENT owners of real '74 360 Duster 4 speed cars on here?

I'm sure my 67 Charger,68 Super Bee,70 Challenger and 70 Cuda will be worth more later on.
I wouldn't be so sure...
The majority the the interested/serious buyers are dying off fast.
Not too mention in 20 years it will be much more difficult to own such an Earth polluter, I assure you of that. Between insurance premiums, dealing with whatever B.S. the Gov't sticks to muscle cars, and gosh only knows how fuel will be then...
There is a real possibility of market saturation. Likely might be only 10 interested buyers for a certain model of car (say your 1970 'Cuda) at any time and there could very well be 35 of them for sale at any given time.
That'll have detrimental effects on the value, no matter what the car is.

It's all about supply and demand...In the 1990's/2000's there were far less of these cars out in circulation and a lot of people wanted them. Thus everything was selling and for good money. As I see it the problem is, now that everyone has drug anything they could out of the woods/swaps there are a ton more driver/restored cars out there than back in the 90's/00's. Great for the hobby, not so much for value. Add in the serious buyer pool shrinking daily as I mentioned earlier and multiply that by 15 years and add in the thought of tougher gov't regulation and higher fuel prices...who knows what will happen.
There is a good possibility nobody will want these old sloppy pigs by then.
After all the 300-550 horsepower fuel efficient New Chargers/Challengers, Camaros, and Mustangs will be old/well used cars by then too offering far greater bang for the money spent.
Think about it, power, efficiency/economy, safety, handling, braking, and will be ~$5k by then...
It'll take a hardcore person to want to deal with a 60-70 year old hoggish muscle car then.
Yes there will still be some market, there always will be.
I just highly doubt it'll ever be like it was in the mid 00's, in fact I think it will still decline or at least will again decline in that time frame you speak of.
Of course I don't know either...nor do I care.
I buy my cars because they are my passion and they are what I want. I could care less if they are worth $65k or $6.5k, I own what I do because they are badass in my eyes.