Don't you think on the fact that GT5 is seemingly "incomplete" gives more all the reason that GT6 will be the game that will "complete" it AND much more? Unless you think that certain features like livery editor is only possible on the PS4, I'm not sure how I understand why you think that if GT6 is for the PS3, that it will be just as incomplete as GT5. If anything, they can only improve it. Plus they did say they decided to save some features for GT6.
If you're gonna go by previous records, first GT title for a new console is always the one with the fewest content and features... no? Unless PD has been working on GT6 since 2010, you can't expect GT6 as a PS4 launch title to be everything you want, with many features and content. So far it's been true that the second GT title for every new console is always the one with more features and content. Only gripe is the controversial physics change between GT3 and GT4. Nontheless, GT2 and GT4 is the more complete game compared to its predecessor.
As an opinion, GT5 may be the weakest in the series for you, but it's fact that it's also the most advance to develop. Saying that, I don't think it's fair to judge PD on the basis of how they handled GT5 for this generation and saying that a GT6 for the PS4 is the best solution for them, in terms of not disappointing us. Perhaps I have too much faith in them, but I believe GT6 will only be an improvement of what we (and Kaz) want GT5 to be. I just don't think that we can have a content blow it like we did with GT2 and GT3 if GT6 were for the PS4 early on, unless of course they've been working on it for awhile.