While I think points like this are valid to a certain degree, they tend to forget that Grand Theft Auto V also released last year and is the fastest selling game in history. This tells us at least two things. One, the gaming userbase on both PS3 and 360 are still actively buying games. Second, lots of games failed to compete with GTA similar to how games failed to compete with CoD in the past or Nintendo games on the Wii and DS.
Now this still doesn't explain everything so we can speculate on other reasons. Lots of big games released earlier in 2013 and lots of them did not do very well. A few of them could be argued that the brand for that particular series is fading such as DMC. There is a reasons a reboot of the series was done and not a numbered one. Other titles like GoW: Ascension and Gears of War Judgment were "awkward" titles to say the least. Both were prequels that pushed selling points as things the series wasn't traditionally known for (like multiplayer in God of War). Others just got lost in the mix by releasing in a crowded first few months like Bioshock, Tomb Raider, Dead Space, Crysis, and Metal Gear Rising. None did terrible, but none of them really took off either (bar maybe Tomb Raider depending on how you compare it to previous entries).
Past that, lower sales in major franchises like Batman, Battlefield, Call of Duty, Splinter Cell, FIFA, etc can generally be explained by the generation coming to an end and consumers waiting to buy those games on next gen systems and/or new iterations in the series specifically tailored to the next-gen systems. And also competition from GTA V most certainly hurt all games that released September and later.
