COD will sell millions of copies no matter if some angry kids boycots it or not.
Most of the people with half a brain aren't getting BF4/new CoD because it's the same damn game as its last iteration.
Look at it like this. If i like the taste of a Big Mac at McDonalds why would i want the taste of it to change when i'm already satisfied with the taste as it is?
Look at sports games. NFL, FIFA, NBA, WWE, UFC etc....they keep releasing games every year. But people still buy them. They only contain barely improved graphics for every year if even that and then improved rosters/teams and some minor new features.
Just like COD. COD series is not trying to be necessarily innovative and pretend it's art and groundbreaking. It's a concept that already works and got millions of people interested in it.
If you'd change Halo games into something that is...say a third person WW2 Medal of Honor-esque Halo, do you really think the Halo fans would appreciate it?
Only bad things the COD games have brought the gaming world gotta be the constant focus on DLC (though i don't mind paying 15 bucks or whatever for a couple of new maps if they're well done considering it's professional and have taken the employees time to work on it) and maybe the focus on matchmaking instead of dedicated servers. Otherwise it's like any old school FPS shooter.
Only things i'd do to improve COD would be to have more focus on dedicated servers (hosted by Activision themselves not players, we don't need more natzeee admins/mods or 12 year old kids in an authority role), increase the maximum amount of players, make the maps slightly bigger, add a little bit more variety to the guns and finally get a complete new graphics engine for the game. Otherwise it's fine as it is. Personally i'm open minded when it comes to gaming. My favorite games of all time are Deus Ex, Age Of Empires 2, GTA San Andreas amongst others. But just because i enjoy games with an immense story, interesting characters doesn't mean that i cannot enjoy some multiplayer FPS with a basic concept behind it.
TL;DR COD fFormula works, no need to change it and haters can't stop the sales but keep on boycotting and playing their obscure indie games and amateur mods of older FPS games.