I've had a theory for awhile that might explain a little bit:
GT5 is more of a benchmark than anything. They used it to incorporate all these new features, and they will be perfected in GT6. GT5 will still get improvements (like Standard to Premium cars), but I still think it's more for them to test their new ideas rather than take the GT series to its limits, even though it still is in many ways.
I think that some cars, mostly the popular ones, will go from Standard to Premium. It has already been confirmed that some cars will get the Premium treatment, it's just a matter of how many. Then in GT6, all cars will be "Premium" in fact, the words Standard and Premium won't even be used to categorize the cars