GT5 won't sell as much as its predecessors, I think that's very probable. The PS3 isn't the software seller the PS2 was. Look at the highest selling games on the console, and imagine how much more GT5 would have to sell to meet expectations here.
Eventually, GT5 will reach a huge LTD, no doubt. It will be bundled and will sell throughout the life of the console. A true evergreen title.
However, golden cow or not, GT5 is on the PS3, not the PS2. It's probably not gonna sell like GT3.
Also, game sales aren't as much about userbase as they are about competition at the time. Sometimes coming out early in a console cycle can be a boon, assuming the library is weak and you're AAA. Like Resistance: FoM, for instance.
I don't need your console war.
It feeds the rich while it buries the poor.
You're power hungry, spinnin' stories, and bein' graphics whores.
I don't need your console war.
NO NO, NO NO NO.