I've read theories that there was a "strategy" to release GT6 on PS3, make a bunch of money with pre-order offers of dozens of exclusive cars... etc, and then release the game again on PS4... so people will have to buy it twice.
I've never read any such theories...and I seriously doubt that was their intention.
My theory is more grounded with my experience in the industry. No game developer would invest a ton of time and money creating a proprietary engine for a specific hardware platform and release only a single title using that engine. Polyphony took an unusually long time to release GT5...they obviously had a very rough time developing for PS3 and, given the success of the franchise on PS2, they had to deliver a level of quality an quantity that met huge expectations. I view their release of GT5: Prologue, which was obviously more of a demo for GT5 than a full release, more as a way to keep fans of the series from jumping ship to the competitor's platform than as a money grab. It is more likely that Polyphony NEEDED Prologue to be successful to buy them additional development time and money to finish GT5.
If Polyphony Digital really was THAT greedy, they could have released GT6 as a Prologue release and waited until the rest of the promised features are done before releasing a more complete GT6. Or they could charge for every new feature they add via DLC. I'll be very curious to see how many of the DLC items they have promised for 2014 will be free.
Sony's strategy has always been a bit longer term than its competitors. God of War 2 came out on PS2 after PS3 was released and was a huge hit because the audience was already there. The PS2 installation base continued growing well into the PS3 console cycle. Sales for PS3, which hit its sales stride later than PS2, is expected to continue for the forseeable future. It just makes more financial sense to release a high profile sequel on the already established platform especially if it is built off of the same engine. Would you call this a money grab?
Also, very few developers want to be a launch-window developer because it is very hard to release a quality project when you spend a large amount of the development cycle waiting for the hardware specification to finalize. Heck, for much of the time, you have to guess what the hardware will be capable of...what features it will support. There's a very good reason games usually look and play considerably better when released closer to the second holiday season after a console's launch.
I personally chose to not get a PS4 at launch...and probably won't get one until the next batch of AAA titles are released. PS4, like PS3, is an evolving platform. The feature set will change and improve over time. At the moment, it lacks a compelling game library and, more disturbingly, lacks media functionality of its predecessor....both of which I expect to improve before I buy one. PS4's launch was arguably handled infinitely better than PS3, the price is better, there were many more quality titles to play than even PS2, and PSN is in a much better place than it was during its early days, but nothing offered is necessarily of the type that would make me say I must have the system now. I, for one, am extremely happy that GT6 is on PS3 as it will likely be what I play up until PS4's release. I also have a huge "pile of shame"...games in my backlog on Steam, Origin, PS3, PS2, Xbox 360, Xbox, Gamecube, so it's not like I am desperate for content.
That said, what I would personally like to see happen is Polyphony releases a quick port of GT6 on PS4 sometime this year (perhaps around the holiday season) as part of Sony's Cross-Buy program. Flow and Flower are current examples of this...if you own such titles on PS3, you can download them free on PS4. What this would give Polyphony is an excuse to release a feature complete version of the existing sequel, polished, and perhaps somewhat visually upgraded from its PS3 counterpart free from the unreasonable expectations that will likely accompany their first full-fledged PS4 effort. To truly do it right, they would let you carry over your career (and any purchased content) from the PS3 version. Some publishers are offering discounts for upgrading from PS3 to PS4 versions of their games (Assassin's Creed IV is one that comes to mind), but doing this for free would certainly end the "money grab" discussion once and for all.