I'm an optimist. These cars are way too cool not to gain in value over time. However, that being said, The ones that will gain the most are C Code, J Code, M Code, Q Code, and of course R Code. Our standard 302 2v and 351 2v cars will likely only see very minor to moderate gains and only for pristine cherry examples either restored or original. Standard driver quality cars should at least hold steady in price and will vary more depending on location and timing. I see this as a good thing for those of us that love this generation of Mustang because that keeps our cost low. The high end cars drive new parts production, and we all benefit from that. Over the last 15 years the number of parts being made for our cars has really gotten much better. Hopefully one day the body parts will be better, because many of them are complete crap at the moment.
One other thing to consider. If you bought a brand new car, doesn't matter what brand or model, and you bought a 71 Q Code Mach 1. Drive them both an equal number of miles for 5 years, doing regular maintenance and repairs on both. Then sell them both after 5 years. Your Q Code will sell for the same or more than you paid for it 5 years previously, and your Brand new car would sell for 15-25% of what you paid for it. Not to mention that your Brand new car won't be anywhere near as cool or fun to drive as your mustang.