I dont believe MS could allow 343i, Turn 10 or Coalition to move on to other franchises and leave Halo, Forza or Gears behind. Sure, they can let them spread their wings on new Ip's while maintaining Halo, Forza and Gears as well. Love it or hate it, NO ONE wants to live in a world without those 3. Gamers everywhere would go ape s***.
Well Uncharted is at least as big as Gears and Naughty Dog is done with it but Microsoft does play it safer and that is understandable.