Only one small problem with that - plenty of people want to travel between Birmingham and Manchester, and Manchester and Leeds. So then you end up with the Birmingham-Leeds (and wider SW-NE) flow as well as the first two flows, all on the same train.
This is exactly the kind of issue that has resulted in XC's capacity crunch - failing to segregate short and long distance flows onto different services. It might be fine to begin with if you have 400m trains, but in the long term it's not a sustainable solution. T
Although, individually, Brum-Manchester and Manchester-Leeds and Leeds-Brum were probably always going to be 200m services anyway. Therefore 400m ought to be oodles of capacity. One 200m set for Brum-Leeds passengers, attached to a second 200m set for Leeds-Manc to be replaced with Manc-Brum passengers at Piccadilly. i.e. exactly the same level of capacity as would be available with the full Eastern leg.
This might have implications for the Sheffield-Leeds section in 3...