DLC packs/season passes will be a big source of where they make money, SoD2/Crackdown as examples (well pretty much how most games are these days) will likely get a DLC pack after 3 months, another after 6 months and maybe a final one close to a year after launch, people will end up re-subbing to play them and have to buy the DLC/season pass for the content at some point down the line.
This approach will also kill off a decent portion of the resale market of used games with less copies been out there which all publishers want.
There will definitely be people who will selectively sub for a month or 2 here and there for specific games they wan't to play but there will also be people leaving there sub active even if they aren't really using in for a couple of months (I've done this with many services either been lazy or just forget and let my WoW account re-sub for another 3-6 months without playing), It could also be said that the most likely people who would sub, cancel, re-sub constantly probably aren't your general day one game buyers and would of waited until the game was in the bargain bin at <$20 before considering purchasing it.
MS should probably offer 12 month packages for $110 (1 free month) or maybe even $100 for 12 months incentives as people will lock into the sub much like with Gold/PS+ yearly codes where even if you don't use any of the services they provide you have already paid your cash upfront.
