Yeah; if MS had let Bungie create something new after Halo 3 I don't think they would have wanted to leave. You would think this would be a warning to MS to give their developers a bit more leeway, but instead we get Lionhead being a Fable factory, Turn 10 working on nothing but Forza (and they even have to draft in another studio because they want annual Forzas), Rare being stuck with Kinect and most recently Black Tusk's new game being canned in favour of more Gears.
Maybe they'll allow 343i to develop something new after Halo 5, but I doubt it. They'll always be able to hire in new talent, but they'll haemorrhage a lot too because the devs will be in demand and they won't want to work on Halo forever.