I don't see adding cars, or tracks, as a fix all for GT7, but it at least shows that they are still backing the game, and it gives hope of that they are still working on the game in other ways, and would hopefully make some more structural changes. Two years on though, and after the Spec II update, I can't see them making those types of changes now, so that leaves new cars and tracks as something to add ongoing interest for some.
As with most games, users decrease over time after the initial release and boom period(if they are lucky
), but I'm sure PD see an increase in users when they add new content. And they want to keep making money, selling games, or at least helping to sell PSN accounts, so drip feeding new content is a way to mitigate the inevitable decline.
That said, even if you add content, I think the choices of the content can send mixed and confusing messages as to the direction of the game.