I don't think many people want GT5 to be rushed but people are concerned that PD has really gotten to the point of diminishing returns. GT on PSP can be seen as evidence of this.
The cars in PSP GT are at least a few years old, indicating that while the models were completed, the team apparently wasn't happy with the physics models (or tracks, who knows?) and kept tweaking, and tweaking, and tweaking. By the time the game came out, the physics were fine, the game looks great, but all the cars are outdated.
Yeah, there are still some classics there that I certainly don't want replaced but where's the 370Z, the new Camaro, the new Challenger, the Evolution 10? They didn't make it in because PD spent too long messing around with stuff that 99% of people wouldn't notice. It obviously wouldn't have been a problem if PSP GT came out in 2007 but it was pretty jarring in late 2009.