Right, but perhaps better the devil you know than what you don't know.
Personally, I think there should have been a small core team among all the Xbox Games Studios which deals with game engines, experimenting with improving them to meet certain internal business needs, especially if vast majority of those studios are focused on one or two primary engines. It wasn't until just a couple years ago that Microsoft picked up additional studios and gained more overall experience outside the MS in-house engines. Maybe this is happening now, so future titles will be easier to create?