Go back to '67 or '68 and say the same thing. You sir, have no idea what the future holds. Interesting there's a bunch of Michael J. Fox's here..
I don't think you know much about the current status of classic mustangs. A 68 mustang GT fastback with a 302, according to Haggerty, has an average value of $22,600. Only once you get into really rare status, like a 68 convertible mustang GT with cobra jet engine, does the price skyrocket. Sorry, but the odds of your car really being worth much in the next 20 years is slim to none. Frankly, the 2015 mustang GT will be a much better all around performance car than the 2014 mustang. Looks OTOH are completely subjective---some people LOVE the look of the '15, some prefer the look of the '13-'14 car, and I prefer the styling of the '10-'12 cars, myself.
But you never know, there might be a single person out there who will long for the days of a coyote powered mustang with a SRA--maybe that person will pay you gobs of money for the car.
The bottom line here is that you have every right to be annoyed by the dealer. Frankly they have no excuse and you shouldn't have been allowed to drive the car off the lot until it fixed perfectly. IMO, do not settle for a repair---demand a completely new door/door skin. THe tricky part at this point is the paint work--do you really want the same clowns who screwed up your door the first time doing it again? It might be worth the gamble if the car isn't taken to the same body shop, but otherwise, you might want to find a high end paint shop and pay for the paint work yourself--any place that does high end exotics or big money hot rods should be able to get the paint job right for you. Living in the Chicago area, I;m sure you can find a good place to get the work done.