Has the series actually become bad, or are long-time fans just bored of the series?

I think it's a combination of the both. The games have gotten lower in quality since Black Ops II/Modern Warfare 3 (in my opinion), but I also can't play Modern Warfare 2 for hours on end like I used to be able to. I think the games have gotten worse and I have also got slightly burnt out.
If IW was released when CoD 4 was it would have flopped immensely. Futuristic games of that nature were not popular at the time and it would have been too radically different from what people already knew Call of Duty to be.