On a like-for-like replacement basis that would mean 50-odd long trains, as the DfT would never have allowed GWR to acquire lots more sets seating 600+ passengers, and a GWML service pattern frozen in time for decades to come - like I said above, no hourly Cheltenham service, Bristol off-peak limited stops, etc. Never mind that some of those HSTs were being used for large parts of their working lives on duties that did not need anything like 500+ seats.
Just replacing the (five-car, 125mph capable - remind you of anything?) Class 180s and relevant Turbo diagrams with something other than 80xs would have resulted in a small niche fleet with all the operational and cost implications that would come with that, would have to be bi-mode for work on the Cotswold Line or anywhere else past the wires and would also need to be at least 110mph capable to keep up with the 387s on the main lines between London and Didcot/beyond, never mind IETs.
If they were to be regarded as suitable for journeys of two or three hours plus, in the way Turbos have been used to Worcestershire and Hereford for a very long time, then why not use this type of train to Bristol, Cheltenham or Cardiff too, in fact, why not build it in long formations as well and use it everywhere in the GWR area?
Which sounds to me rather like the IET fleet.