I think there are two very important factors as to why GT5 won't be as successful as previous GT games.
1. GT has real compitition now. Even GT3 and 4 didn't have high quality racing sim games to compete with. Now there are several, Forza, Dirt, NFS Shift to name a few. Racing fans have already gotten their sim fix and are less likely to feel the need for more speed.
2. Racing games aren't as popular as they used to be. Sure there are still plenty of die hard racing fans, but the casual racing fans who drove GT sales on the PSone and PS2 have either moved on or found solance in other racing games of equal quality.
I'm not going to venture a guess as to what GT5's sales will be in it's first week, but I think it's unlikely that it will outsell GT3 or even GT4.
