First off, you need to admit and acknowledge that a car, especially a common car like a Mustang GT is not and will never be an 'investment'.
Second, while I feel you are right in expecting to send your car to the dealer's body shop and receive it back undamaged, I think you are going overboard with the dramatics of the paint 'never being right again'. A good body shop can repair a car to showroom condition, with no issue.
Third, in less than a month of owning it, you have put 700 miles on it. That equates to 8-10,000 miles a year. That's basically daily driver status. If you think you're going to put 10K miles a year on a car and not pick up dings and scratches, you are severely mistaken. So if the car is going to receive dings and scratches over the course of the next year, whatever issue that may exist in your head regarding the paint not matching will be moot.
Fourth, the dealer is only on the hook to 'make you whole'. That means restoring your used 700 mile 2014 Mustang GT, to the condition it was in when it rolled into the body shop. They will not and you cannot make them switch out your car for a brand new car. Most reasonable people will take as fact that a professional body shop will be able to restore light damage to a car to showroom condition. So their offer to repaint the door is a valid one.
I wish you the best of luck in getting the situation resolved to your satisfaction. But I think you should establish reasonable expectations and allow the dealer a chance to make things right for you.
My thoughts exactly. A car that has been owned for a month and driven for 700 miles will not be replaced gratis for a new one. They will likely be happy to take it in trade for another one letting you take the full hit on the depreciation, which is more than 700 miles at any reasonable cost per mile.
2014 GT Premium Convertible - Black / Charcoal Interior, Auto, Brembo Brake Package, Rear View Camara