Final install base of PS1 and PS2 is irrelevant, because the back ends of those generations were occupied by GT 2 and 4, while 1 and 3 are the games that sold better, releasing to smaller install bases. Sales of GT 1 and 3 were probably cut off somewhat by releases of 2 and 4 respectively. I doubt GT6 will be released for PS3, or if it is, it will be at least 3 years after GT5 is released, giving GT5 ample time with a sizable install base. And looking at the sales of Prologue and considering how vastly improved GT5 will be -- over Prologue, and yes, over Forza 3 -- there's really no reason to think GT5 won't reach 8 million.
GT sells mostly to casuals? Try, and ultra hardcore racing fans. You need to meet up with the 360 fan on this site who just today posted that GT5 will fail because it's not hardcore enough for the hardcore fans that have traditionally bought GT games.... between the two of you, GT5 can't win it seems. Until it does.
And face it, Sony's Halo equivalent (though slightly bigger than Halo) hasn't seen a full release on PS3 yet.
