The point is that Grid and Shift are not comparable to GT. They're much more comparable to Hot Pursuit and Most Wanted than they are to GT. If you want a realistic driving experience, you do not choose Grid or Shift.
If you want a realistic driving experience on PS3, GT is it. Don't get me wrong, GT has done a lot of things very well in the past to get to where it is. But on PS2 there was at least competitors like Sega GT and Enthusia. On PS3, there is nothing. GT is a monopoly, with a few semi-simulator games like Ferrari Challenge being very careful not to step into GT's space.
Why that should be I'm not entirely sure. For a game with sales like GT I'd have though there would be companies falling over themselves to clone it, like there are with GTA. http://en.wikipedia.org/wiki/GTA_Clone
GT stands more or less alone, and it's certainly alone on PS3. How much of it's sales are down to that is debatable, but if you want a realistic driving game with a range of cars on PS3 you buy GT or you buy nothing. There is no competition.
Maybe because it isn't easy to do what PD has accomplished with the hardware? You may see clones of GT6 for the PS4 now. Easier to write for and powerful (RAM) enough to make the job much easier...
Given what PD could give us on the PS3, it'll be interesting to see what they can do with the PS4, with fewer "hoops" to jump through to get the hardware to do what you want.