What's the point in that though? So they moan about it getting stale but then say they want it to go back to how it was? Eh?!

There's no pleasing some people.
thats a whole separate crowd.
I know the people you're talking about - the ones posting about "Call of Duty Recycled Warfare" and the only way for those people to be happy is for activision to abandon yearly releases and release one COD game every 2/3 years and then brief the developer to try and make every game different from the last.
Trouble is, the majority of the COD fan base, who actually buy these millions of copies every year, don't mind the game being broadly the same. Change it too much and it won't be COD any more. A change of theme would be nice (eg do a Pacific Campaign WWII shooter) but again you can only change the theme so many times. Change it every year and you'll run out of themes. Activision can't win with this lot.
But in my experience the "Recycled warfare lols" crowd are usually the one that haven't bought a cod game since MW2 and say they won't buy another until they use a new engine or some other daft criteria. Even if they did change the engine, and re make the game from scratch and properly shake things up, the next game after that would be broadly similar and the complaints would start again. You can't re innovate every 12 months and there's no point trying only to please this demographic, especially when this demographic aren't buying COD games every year.
Most of us in this thread do buy the game every year - getting broadly similar games is bound to happen when you release a new one every 12 months. What should activision do when they make a perfect COD, never release another COD game ever again/ How do you improve on perfection without breaking it ?
Most of the core fanbase that bought every COD game every year and didn't buy ghosts - has done so because they've got fed up of the current trend for modern futuristic shooters full of BS and overpowered weapons and scorestreaks.
Keep it similar by all means, but focus on making the game simpler, and more balanced, and remove all the BS stuff designed to let people who are terrible at the game get revenge on those that are better through cheap means - like dead man's hand.