I think you're looking at performance. When really the bane of slipstream is iteration and tooling. They can hire as many people as they want at 343i, but the ability for everyone to make changes to the game simultaneously appears to be the bottleneck. Bungie indicated this with Destiny, it can take a whole day to recompile just to move a garbage can. I don't know if that's been solved, but unreal engine makes its changes in real time with lighting etc, and now with DXR (I'm not sure if yet available), you can pretty much make the whole game without baking lighting once in the editor with pretty good confidence of knowing what it will look like. The time savings would be massive.