Even for the PS4, attempting to make all cars in the game 'Premium' would be a very time consuming process. Unless we wanted GT7 to release in another 5 years time, the only thing PD could realistically do is upgrade the 'Standard' cars so they are satisfactory for the PS4. I couldn't care less if half of the cars in the game were lacking interiors. It's the fact that many of the cars have exteriors of a simply unacceptable PS2 quality. I will be more than happy if this is resolved for GT7.
It's not like this is a huge demand. I'm not an expert in game design, but going back to each 'Standard' car in the game just to polish off the exterior cannot be a hard or extremely time consuming thing to do. They already have the bones of the construction. It's just a case on upgrading the flesh as it were. In other words, smoothing out jagged edges and upgrading the textures, so the car is of a semi-premium quality. It would save a lot of hassle, and would make all the cars in the game look incredible, despite many of them lacking interiors.