I'm not sure about the demographics of videogamers across the globe, but I would imagine the number of gamers doesn't grow too rapidly. The number of gamers a company can sell to is more finite than one might think (again this is theory because i'm too lazy to do the research for a forum post).
Perhaps I'll use a real life example to help my point (one that I am writing while exhausted and not sure why I'm trying).
At one point in my life I sold things to small businesses for a living. I've also sold things to colleges and university in the United States. It was easier finding new customers to sell to in the small business world because new businesses were always starting up. New prospects popped up on my radar faster than popcorn in a microwave! On flip side, there are only so many schools in the U.S. and its not like there are new colleges starting up on the daily. This means sales eventually slows down over time because you run out of prospects to sell to (until you have new products/services to sell).
I would imagine the videogame industry is similar. More specifically, the demographics around people that enjoy RPG's. It's more of a niche market and I would imagine there aren't tidal waves of new gamers pouring in month over month. Sales will slow down over time. Of course the numbers will look amazing closer to the release date and not so much as time passes.
TLDR: Sales will eventually plateau in this type of market.