It all depends if they release a great game or if they only put out thurds, just like any other franchise...
Let's take two recent example: Grears of War Originas and God of War Ascension... both showed declined sales for their respective franchise, honestly I think they were both low point for their own reasons... so whatever comes out next in their line up will decide if their respective franchise is succesful or no!
Same went for CoD Ghost, it was not very good, did not sell as much as previous CoDs... it seems the new one is doing better, because it's a better game, it does not matter if it has CoD in the title, not for everyone at least.
That works for every franchise it can be in games or cinema, or anything really... there is a core of followers, they will keep on buying the installments a few times even if the new titles are bad, probably out of abit... then if a few more good or great titles come out they sell a lot with the name recognition + good impression, because more casual admirers want in on the action when it's good! or if nothing is made they fade into oblivision (when was the last great Mega Man title released?)
