Of course it is, and to argue otherwise is just silly. It's like with the lives of humans, wherewith you spend the first part of it growing up, growing bigger and better, and then you spend the bulk of your life withering away until you cease to live any longer. GT5 was released in late 2010, its installed base grew and millions of people played it for a while, and then after a few months the number of people still playing it started dropping off and continued doing so to date.
I don't have precise figures and it's probably pretty difficult measuring the number of people actually playing without settling for simply measuring online activity, which would never have been indicative of the full number, but I'd bet good money that within six months of GT5's release the majority of players who had bought the game had by then moved on. By the end of 2011 it was probably more like ninety percent had left. Now we're roughly two years from release, where it should just be a dwindling number of stragglers who were unable or unwilling to buy an Xbox.
"Hey, I said not to mention Forza!" I didn't. I just mentioned a very relevant point, and it
is irrefutably relevant there. To avoid it is to avoid the facts and to fail to adequately address the topic question.
"GT5 isn't dying for me." That's fine, but doesn't matter. The topic question wasn't whether it was dying for me or not, or for you or not, but simply "Is GT5 dying" and the answer is obviously "yes". Some players playing doesn't change the fact that the vast majority have left.
"True GT fans would still be playing." Only if they don't own any other game consoles. A great example is my buddy Terry, who has long adored the GT series and boasted about the enormous amount of time he spent with each; how he played them for years apiece. He expected to do the same with GT5. He moved on months before I did, and I moved on by about ten months from release. If he was into tattoos, he'd probably have a GT tattoo on him somewhere, and he still didn't even stick with GT5 for as much as a year. Clearly then it isn't a matter of how loyal of a fan you are, but more of having other things to be doing; newer things and even some better things.