The truth is, FFG only has this license for a limited time. So I imagine the smart business plan is exactly what they are doing. Releasing new games with the same general system and different fluff with some alternate style rules. Target it at the already existing audience and make as much money as possible until GW takes back the property they own and publish it themselves.
I couldnt imaging the contract lasting longer then say 5 or so years before it needing to be renegotiated or extended. So I think Black Crusade is the beginning of the "we are near the end of the contract" cycle. So yeah, I think we will see a new game line every year until the contract is renewed.
The first thing, I imagine, for the lines once a extension is granted, would be a universal core rule book.
Heck, at this point in time Id kill for a conversion book.