keep in mind that Walmart has not been counted in the past, so in actuality, their sales could be less now then what they have been in the past. I dont think that shooters are a fad, what I do think is the COD franchise is on the downward slope. When the series makes one too many(like it will) and another competitor or series advances the fanbase will move on. Activision/Blizzard just see the $$$ and dont care about content. The moment they overstep this tired franchise and milk too many is when the series will go belly up. I predict that when this happens, it will have an impact on game sales.
The problem with the industry right now is that sequels are the norm. Instead of having a sequel every now and then, we have one pumped out every year. The revenue result from a successful sequel is too much a draw for several companies now. In the past, we had a game come out and a lengthy time in between games. This helped generate hype and create demand for games. It just seems now that every year we are to expect yet another game from X franchise game, paired with portable X version of the game, combined with smartphone X version of the game.
At some point, people are going to express with their wallets, that they have had their fill. I expressed this years ago, and still believe that there will come a time when the industry will go to the well of a franchise and it will not be as full as it once was. What goes up, must some time come down.