As already pointed out we do not know who is to blame for the fact that GT5 isn't out yet. Could be Sony for strategic reasons, could be Kaz because he isn't satisfied with what he got. The OP seems to think it's the former, I do think it's the latter. PD, as a developer that has based the rep of an entire franchise on their groundbraking graphics, "almost-sim" physics and game-size (number of cars, tracks, gt mode), and remained head and shoulders above any competition in the PS1 and PS2 era, is in this generation of consoles facing competition more fierce than ever.
PD, always focused in the graphics quality of their products, started to fall behind their still shy competition when the PS2 console life was approaching its end. GT4 is/was the swann song of the traditional GT "natural evolution". Graphically, you couldn't squeeze more than that from a PS2. But other game companies and devs were already setting their sights and doing better than PD in other aspects, such as physics (Enthusia), online (TRD3 and TDU), damage (TRD3), larger car fields (TRD3 again). GT4 still could beat them all (and it did) but I can only imagine how PD must have felt the need to "up" their game. Especially PD must have known by then that GT4 would forever remain as their last offline-only game.
I'm not sure (none of us can be, with the possible exception of Amar and other even more silent members) who is to blame for all the
zigzagging that happened later. I really don't know if
GT Vision was really nothing more than a trailer, or if indeed PD made a HD GT4 with more cars per race (and online-able?) with the possible intention to release it for the PS3. THen we had GTHD announced, with premiium and standard versions IIRC. And a litle "demo" available for download on PSN. Still not online-able.
After, we had GT5P, and what was supposed to be nothing more than an appetizer to a full game (like GT4

was, released months before GT4) has had to do the honours (and it did, valiantly I might add) for a period longer than many other game's life cycle.
GT5P, however, has shown us exactly where PD had in 2007 still a lot of work to do. Online is the poorest you can find, even worse than what TRD3 already gave us in the PS2 era. Damage was still nowhere to be seen so was changeable weather or day/night cycle.
Since the release of GT5P what has PD been doing? My best guess is that apart from the development and release of GT-PSP (and that we can assume was Sony's demand), what really occupied PD was simply this: "how to make a game that respects GT's graphic quality standards and tradition while at the same time achieve fully functional and also "perfect" ONLINE, DAMAGE and WEATHER features.
And if you guys notice:
1 - ONLINE - we know nothing about the online features (apart from the promises of private lobbies and the glimpses we had from the LAN play reports and pics we got from the participants in the GT Academy National Finals);
2 - DAMAGE - All we got so far are very unclear reports about two kinds of damages implementation, and all we saw so far in clips is very, VERY poor, far behind well established competition (namely from codemasters just to mention the best I've seen so far regarding visual and mechanical damage). In fact, all we saw to this day is so poor it is worrying.
3 - WEATHER (
random and
changeable or
fixed and
the same from start to finish) and DAY/NIGHT (with or without cycle) - I didn't see anything truly significant so far.
So ... maybe all this is now - finally - ready. But, back to the question put by the OP, my belief is that PD (and Kaz) is the one mainly responsible for such a long period between the release of their last "full" main title (GT4) and GT5.
So, in short, I don't think it makes a difference if Kaz leaves Sony (and PD, owned by Sony and not by him) to start his own studio. Indeed, I think that would probably mean we would never see a GT6 being released.
Sorry for the long post, when I started it I didn't mean to write so much