If the heavy maintenance of the 175's could go to Longsight, then TfW could keep the 3-cars as well and use the Swansea-Manchester route to cycle the units between Landore and Longsight.
It would be a logistical nightmare for TFW, it would be an all or nothing lease agreement . when the 175 units were shared at the start of the ATW franchise. There's was multiple empty stock and unit swaps at Piccadilly and Oxford road.
The congestion at Manchester is far worse and the impact on other tocs would cost TFW a fortune.
If TFW were to retain the 175 units, a new heavy maintenance facility would be needed close to were they will be working. The units work well if they are well maintained, it's when the maintenance gets skipped the reliability drops Alstom have found.