I personally thinks it's taken too long. GT4 came out in what, '04? And GT3 came out in what, '01? So why does it take twice as long? They must have started work before the PS3 came out, due to their affiliation with Sony.
Prologue is like a beta test-bed for PD.
It would make more sense to compare the year they released GT2 to GT3, rather than two games on the same console. Bear in mind, though, the bar they've set themselves to beat (and so need to have as many cars as GT4, and probably more tracks), the fact that every car has to have an interior made for it as well which is probably more complicated than an entire car model on GT4, then there's the car model itself which is definitely several times more detailed than GT4, AND its their first game on the new system. GT4 already had a lot of the work done, I'd imagine, thanks to GT3, and GT3 and 4 were on a dual layer (I guess) DVD, whereas GT5 is on Blu Ray, and so they have a lot more space to play with.
Oh and I guess they want to get it right the first time so they're letting GT5p run it's course for a bit, recognise trends and patterns and how they start to form and what's popular and what's not, what works and what doesn't (Elise Tuned Car being toned down, for example), and all that, so basically I think they're just trying to make an amazing game, and that takes time.
On topic, though, I'd guess that they have the UI and structure 95% done, but I'd hope they've only got 40-50% of the content finished, as in cars, tracks and career mode races. Think about it, if it's not coming out til 2010, that could potentially be a LOT of stuff to play with!