I disagree with this, I think GT7's track list is better than GT6's in every way except for the lack of city courses. There's more tracks, there's more layouts, the tracks are all much higher quality, there aren't any PS2 holdover courses, and the fictional circuits that debuted in the PS3 era were by far the least interesting; they all felt empty and samey, like they came out of the GT5 course creator.
Edit: Also, "takes too long to make 4K assets" is why every HD video game in existence takes years longer to make these days, it's not just a Gran Turismo issue.