GNER introduced 2+9 HSTs well over 10 years ago which would have delivered a bigger capacity increase than 70 seats.
And your point is what? An 2+9 East Coast HST seats 540-550 people, so about three dozen shy of a GWR 2+8 high-density set, and 100 shy of a nine-car or 2x5 IET.
The fact that 1x9 and 2x5 have the same number of seats tells you why coupled units are such a bad idea - as well as extra staff costs you lose a whole coach to kitchens, cabs and disabled toilets. They should all have been 10car units with an increase of around an additional 90 seats on top of the current 9 car.
Not this tired old one again...
Yes, let's replace all the single-unit 180 and Turbo workings to Oxford and the Cotswold Line with a 10-car monster seating almost three times as many people.
Let's run a 4tph service to Bristol and work it with 10-car sets, providing 3,000 seats per hour each way.
Let's step up the London-Gloucester/Cheltenham service to hourly and then run it with 10-car sets, instead of alternating HSTs (that aren't noted for being packed out off-peak) and 15x dmus to/from Swindon.
Let's get a big dose or realism here about the various types of stock the IETs are replacing and the different needs of the routes they will be working - they aren't one-size-fits-all and that size most certainly isn't a single-unit 10-car train, boasting more than 700 seats.
And let's see what capacity requirements look like once the full new timetable and GWR train fleet is in place (plus Crossrail in the Thames Valley at the end of 2019) - growth in passenger traffic clearly slowed down around the network in the past year.
How anyone can say it is probable that more coaches will be ordered beats me. Most of the fleet GWR will have to work with has been ordered directly by the government, using a complex and costly contract that it is going to be extremely reluctant to revisit to make it even more expensive.
