Once a franchise reaches that mega level it sells based on brand rather than the individual quality of any one specific entry. That goes for any big brand. Call of Duty, Halo, God Of War, World of Warcraft, The Sims, etc.
Now what may occur is that after a few bad entries word of mouth starts to decay and the brand starts to decline. Time also in general slowly tends to erode any brand. But as far as sales, Microsoft has nothing to worry about whenever they drop the next gen version of Halo. "Regular" people don't buy games because of devs. No matter who the dev is. The same way that "regular" people don't go see movies because of directors.
Now from a gamer perspective it is reasonable to care about devs because we care more about the quality of the game. So as long as people are able to separate those two distinctions, everything is cool. The problem is that the "hardcore" often aren't able to do that.
