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

Are more recent COD games actually bad or are we just burnt-out on the COD formula? If Call of Duty 4 and Infinite warfare switched places, switched when they were released, which one would we be calling one of the best in the series? Or do you think there are fundamental differences in game mechanics that made COD4 better than IW? As someone who has been playing since COD2, this is something I think about every now and then.