You may notice that you don't need a devkit to do
research,
talk with manufacture,
scan and create cars
test and create + discuss some feature,
Record and work on sound
License cars
And so on.
Research is pretty general term, and there's a number of facets for researching that might be restricted to needing the dev kit. Since there'd be no point in looking into something without knowing it's implemented limitations.
The same could be said for testing, creating and discussing your feature sets. I would think that a system's hardware specs would have to be present to see where the limitations lie. The same goes for working on sound, you could make a whole model on how sound is implemented (assuming they even want to change it, we all hope they do, but their hands might be tied), and it might be rendered obsolete or ineffective on the new platform. Again, best to have a dev kit around for doing something like this.
Then there's the simple fact that a plan and road map for GT6 could've been laid out several years ago to ensure that they made the most of the physics model they spent all that time creating for the PS3 to ensure that it doesn't only get used on one main title.
Also, I'm not so much arguing with you.. more along the lines of me just saying that we don't know what they do or do not need a dev kit for. 'Cause that would also require us to know what features they want to do, not to mention the possibility of a GT6 product plan existing long before the PS4's debut was ever finalized.
Yamauchi said a few weeks ago that the reason for less GT5 updates is because they focus on GT6.
Well in 2010 they start working on GT6, bur we had big updates and so on. This means they really work on GT6 since 2012 or holiday 2011.
Fact is:
We don't know on which plattform GT6 will be released. Both is possible.
I do agree to an extent. I was merely stating my case for the possibility of a release on PS3. Most of this comes from research I did over a year and a half a go while arguing this very same topic in the GT5 section.
A number of sites I visited provided insightful quotes that, at the time, suggested that a PS3 release far over a PS4 release, because the PS4 was still a twinkle in Sony's eye at that point. One of the quotes specifically was Hirai-san saying that he expected the PS3 to have a similar 10-year cycle to PS2, which would mean the PS4 would pop up around 2015-16.
Now we don't know how well PD is in the loop around Sony HQ (I'd imagine they'd be quite a bit given Yamauchi-san's position within the company and the sales success that the GT franchise has been for all playstation systems it's been released on.) But if you're hearing the boss essentially say "PS4=2016" and you're telling your players "You won't have to wait as long for GT6"... then you'd better be start developing a game for the PS3!
Obviously with all the hot rumors of the PS4 dev kits circling around, things have changed a bit, so we'll see. Hopefully TGS can enlighten us.