The fact is that GT5P is a (glorified) demo. And it still sells this well, even though people knew full well the full game was coming (even if no one knew when, at the time.).
Forza might not have been out for as long as GT5P, but again the point is that a Glorified Demo such as GT5P sold this well even compared to a full entry in the direct competing franchise.
My point being that GT is still a very strong franchise and has not lost its appeal, unlike what many here tried to suggest