May they suffered a bit of the same problem TB described in regard to sacred 2 (if my memory serves right). The development started a long time ago and they may have done if not wrong but not optimal choices when they set the basis for the engine of the game. 5 years ago things like spurs and plenty of other pre made libray were not available. They may be when they realize they were facing issues it were too late in the development. But for such a game I don't that that would explain the whole story. I suspect some management errors, the multiple delays are a proof of that.
It looks like they tried to fix things, failed, delayed, fix again, etc. At least that's how it look by the communication surrounding the product launch, "will launch when ready" two "prequels" releases.
At some point should have consider a technical "reset" announce a "huge delay" like the game will be out fall 2010. There was a lot of pressure on the team I guess, it's tough to start on clean basis when you have to release "prequels/demo" (I speal of GT HD / prologue).
But that for the technical aspect I wonder if something else went wrong during the project, exec loosing faith or patience? I don't know but I'm surprised about the content too, old tracks barely reworked, idem for 800 hundred of the cars available. There is something fishy. GT team has to be big either artists don't have proper management and/or tools or the team was not that big or was missing resources of some sort. I really wonder if at some point execs put a stop realizing that may be the way Polyphony was working on the ps2 wasn't scalable to the bigger team/resources a next gen requires and it was close to a cluster-fuck. The exec may have say stop and you have one year to ship and put whatever you have together.
EDIT
I also see an interview with Kazunori Yamauchi a game related tv channel (game one) and he spoke quiet a lot of errors he made during development (mostly thing he should have left out of the game to be implemented on sony next system for example). It sounds a bit weird, I know the man perfectionist never happy but still the game has just released I don't expect him to be like the forza devs that were I would dare to say "full of them self" "overly poud" while speaking of their work. Really I smell something fishy. He also said that the next gran tourismo were to ship "fast". I would not be surprised if he feel like he has to redeem him self of some lacking of the product.
Overall GT6 shipping in two years would not surprise me. They put ninja to get a new engine on its feet in two years, refine their assets and launch.
Sometime more can be less, 1000 cars is not needed to make the best racing game, as well as kart, nascar, etc. I would not be surprised if GT6 is more focused to its business which is GT racing. Actually I expect a lot from GT6, lean interface, less cars but better, outstanding graphics at 720P, they may even consider 30fps if they manage to get the input lag as low as criterion in NFS, huge step in AI, impressive physic and damage model, flawless online.
