I don't know - BF4 doesn't look enough of an update from BF3 to me. They could have just done everything that BF4 adds in a major patch to BF3 rather than flogging the cash cow and releasing a whole new game. Ah well, that's EA for you.
I almost always get that sense, Call of Duty wise, but I dunno, the destructibility element to maps was probably way beyond patching (I don't suppose it matters to a lot of people, but I like the idea of everything being destroyable) As you point out, though, releasing a new game to the series every year is a very compelling prospect for studios.
As 'tired' as the COD series is I suspect they've got a few more years to milk - Especially now they've got a next-gen platform for their release this time next year.