I've paid for all the DLC as well, but I completely disagree with you. That would be absolutely ridiculous to not bring in the cars/tracks that were included as DLC in GT5 in GT6 just because people had to pay for it. Let PD include all the cars/tracks that were DLC as part of the standard car/track list and make completely new DLC for GT6.
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I still think that all the cars/tracks that were included as DLC will be part of the launching lineup on GT6. Just look at some of the cars they released. I really don't believe that PD will keep cars like the BRZ, Corvette Stingray, Aventador etc. out of the lineup (or limit it to just those who purchased it previously) just because it was DLC. This is a new GT and I'm almost certain that they will have them all in at launch (minus the TC's possibly).