Wales has to be a beneficiary from this fleet cascade in some way. The current ATW fleet is stretched to crisis point.
The DMU's being released can provide instant relief in Wales:
The 153's and 156's can go to the valleys to help replace pacers. The 170's can be added to Wales' long distance fleet to supplement the 158's and 175's, which in turn could also release 150s from North Wales to help Pacer replacement on the valleys.
Longer term, if the WG can be persuaded to abandon the proposed tramway conversion for the core valley lines then the 321s can provide an instant ready made fleet with plenty of capacity if refurbished to a high enough standard.
After all, the original business case for the valley lines electrification in 2012 was based on cascaded class 313s providing the initial fleet of EMUs with new build stock following later on.
Even if the core valley lines are converted, heavy rail EMUs will still be needed on the Vale of Glamorgan, Ebbw Vale and Maesteg lines.
I think that ATW would benefit more by being indirectly involved than directly involved. It'd make more sense to send the AGA fleet to TOCs with existing fleets of the same type, allowing them to release units to ATW which they already have examples of. It is cheaper and less disruptive that way.