They could go 3rd party, but what would be the point? Microsoft has nothing good to offer.
-They don't develop talent
-They don't allow creative freedom.
-They run their studios into the ground.
Halo, Gears, Forza are former shells of themselves. Avowed was luke warm. Starfield was terrible. Perfect Dark was canceled. Theres no word on Fable. Only Indiana Jones is the game that should signs of good.
I say good riddance to Microsoft. Let Valve take their place.