My view here is what happens when the world changes again, and someone decides 5 car trains are all we actually need? Away with the 9ers and 7ers, it'll all be 5ers.
An unlikely situation, granted, but let's say we roll with it, and have nothing but 5ers. The depots are all reconfigured at great expense, diagrams changed, platform workings altered and all that, then a world event arises and the ultimate decision is made to go all over to 5ers and 10ers only.
Another expensive round of changes would have to happen. It is unlikely such a world event would happen, certainly in the lifetime of these trains, but then who actually thought we'd have the world event we did have arise 2 years ago?
I say leave the IETs as they are, all the costs and efforts to reform them just don't make sense. Surely providing more capacity is a good thing? I'm certainly not ready to return to overcrowded, short-formed trains. We have enough of that on TfW, we don't want it on GWR or, indeed, elsewhere!