Personally I believe the decline in sales is due to no new consoles coming out these past years. When everyone who wants a console buys it the first year it's out, it's going to be at it's highest sales. The next year when it drops in price and friends talk friends into buying it, sales will be lower but still high. Then the next year.. and the next year.. and, well, who's left? Sure there will always be those people just behind the times, or just now coming of age, or having to replace broken consoles, or buying thier alternative choice system, etc... but that's a much smaller margin of consumers than the fanboys and thier friends.
Blaming it on sequels alone is kind of ignorant in my opinion, especially considering sequels make up the big majority of gaming sales. Can't think of a single non-sequel to come even close to selling as many copies as Call of Duty 7. Not to mention FallOut 4, Assassin's Creed 3, Final Fantasy 13, Halo 5, need I go on? Hell, the only game I can think of to make good sales was Red Dead Redemption, which could easily be argued to be Grand Theft Auto 5 (techniquely 9!).
Blaming it on sequels alone is kind of ignorant in my opinion, especially considering sequels make up the big majority of gaming sales. Can't think of a single non-sequel to come even close to selling as many copies as Call of Duty 7. Not to mention FallOut 4, Assassin's Creed 3, Final Fantasy 13, Halo 5, need I go on? Hell, the only game I can think of to make good sales was Red Dead Redemption, which could easily be argued to be Grand Theft Auto 5 (techniquely 9!).