It might have started development first but it wasn't done until 2010. You have no idea when the graphics engine was finished and fully optimised but it surely wasn't years before. GT6 has not been announced yet, they are not going to announce and release a AAA game within six months. Secondly everyone and their dog knows PS4 is going to be announced at E3 at the latest. Do you really think they're going to announce a next gen console then announceGT6 as a PS3 game? No.
I really don't know what you're seeing in that trailer, it's all the same as other GT5 videos pre-rendered. Watch the E3 2009 video.
GT5's engine was complete in 2008. After Prologue it was all content and fixes. The only changes to the graphics engine from Prologue were things being tuned DOWN for optimisation purposes I can only assume. None of the Sony 1st party studio breakthroughs for Killzone 2, Uncharted 2, Resistance 2, God of War 3, Heavy Rain had been fully developed in time to be integrated into GT5 at all. None of that stuff hit till 2009, which is when GT5 should have originally come out....when graphically it wouldn't have looked so bad. But to come out close to 2 years after that stuff is what makes GT5 looks FAR less impressive.
Imagine taking GT5's current graphics and content and properly integrate the kind of texture and anti-aliasing tech from GoW, Lighting software from Uncharted/Last of Us, memory allocation and real-time physics and particle technology from Killzone......this stuff is shared with Sony studios, and is fairly easily integrated into existing engines when they are "opened up".......Right there you have visually a product that looks leaps and bounds beyond GT5. You throw in some more tracks, some new car models and tweak some UI, and there you have another 12 millions units sold with GT6 on PS3.
The alternative is once again new hardware to have to figure out with a gun to your head, and a smaller install base...which means less profit for your hard work. I think a last minute GT6 on PS3 makes far more sense on various levels.
As to your other point, yes, at the end of a console generation you can easily announce a AAA game 6 months before it's released. Both console have 80 million potential customers that don't need more than 6 months to plan to buy a game. And it's not that strange at all for a game to be revealed at E3 to be released that following holiday season.
Also, you don't know for sure PS4 is going to be revealed at E3. And even if it is that wouldn't change any plans through early 2014 for PS3, a console would CERTAINLY not be one of the cases where it's coming out 6 months after the E3 reveal. PS4 won't be on the market till Spring 2014 at the earliest and more likely Fall 2014. There's no reason Sony couldn't reveal GT6 for PS3 (especially if it's really impressive, and won't be a long wait) to come out before Christmas 2013, and also reveal PS4 with a date some time in 2014 at the same E3. I don't think the two would affect each other. Especially if PS4 is backward compatible....which I can't imagine it wouldn't be.
Remember, there are still a TON of big games coming this gen. Alot of which don't even have dates yet but we know will be current gen. Then there's THEIR DLC cycle......... I don't think this gen is in reality going to end as quickly and abruptly as you and alot of other gamers seem to think is going to happen after this Fall. I'm not expecting the new hardware to hit market till Fall 2014. Nintendo didn't come strong with Wii U.....it really is catch-up hardware contrary to what alot of fans on the web say. So Sony and MS are under no pressure for new hardware from Ninty, and I don't think there's any pressure from us gamers for new hardware....only expectation from us. The only pressure for new hardware is from a few high end developers.
To cut my typically very long posts short.....there's still PLENTY of time for GT6 to be on PS3, and I believe that will be the case. Kaz said both that GT6 work began right after GT5 was done (probably because he was pissed he was getting critiqued for graphics, so wanted to start encorporating new tech into GT5's engine ASAP), and he also said that GT6 development wouldn't take NEARLY as long as GT5's which was 5 years. Well this Fall will be 3 years...............
My logic is making sense to me at least.....as convoluted as it may be.