There are an incredible number of details yet to be revealed about GT6. We don't even know if there will be used cars again. Of course...
We don't know what the Standard cars and tracks will be like, but that doesn't prevent people speaking with pre-knowledge authority that they will be ugly primitive last gen crimes against nature.
We don't know what the final audio library is going to sound like, but that doesn't keep people from stating that they will definitely be the same old pencil sharpeners as in GT1.
We don't know what the final tire model will be, but some people evidently know that the same rubber will meet the road as always.
We don't know what GT6's A.I. will be like but... well, you know.
It wouldn't be Gran Turismo or GT Planet if people weren't roundly criticizing it ahead of time. But of course they all have the final release of the game, so it must be true.