The problem there is that the season is most of the year now. Incidentally, There seems a great many 9 car trains going to Worcester and Great Malvern. Are those stations busier than Plymouth to Penzance I wonder during non seasonal periods. I doubt that. The splitting of 2x5 can be used both ways as an argument.
No plenty of 9 car off peaks running on the cotswold and to and from oxford by no means restricted to the rush hour.
How many more times do we need to keep going over the same ground?
We did this only the other week, the last time that irish_rail mentioned yet again how unfair it is that no one wants to run half-empty 650-seat trains all the way through Cornwall for large parts of the year. Please don't pretend that the number of people travelling west of Plymouth from September until just before Easter is on a par with the high summer - when the rolling stock allocations are altered to reflect the increase.
On that occasion, I pointed out that the trains that are "by no means restricted to the rush hour" almost invariably reach Oxford from Hereford and Worcester at a point in the afternoon which is the peak, by any rational definition, with large number of people heading to Reading and London boarding there, and that when they eventually get to London, they immediately form peak workings back out again - but apparently that's not stopped the same 'economical with the actuality' line being trotted out yet again.
The rough percentage split on current Cotswold Line IET allocations is that 60 per cent of services are worked by five-car sets (basically former 180 and Turbo workings) and 40 per cent (running in the weekday peaks and the busiest parts of the weekend) are nine-car sets (mostly ex-HST duties, as cactustwirly says). And that's not likely to change much with the new timetable.
Can we please stop pretending that everyone who gets on a London train at Penzance or Truro and vice-versa is going to stay on the train for the entire journey and wants to eat in a restaurant car? A number of posters on this forum may be deeply obsessed by Pullman restaurants - by and large the average passenger isn't and will never eat in one, even when the opportunity is there.
The service in Cornwall is being overhauled over the coming months, with some of the extra regional services starting from May and then a regular 2tph service both ways coming in with the new timetable, giving more options for local journeys and taking pressure off the longer-distance workings as a result. There is no prospect of 2tph all day both ways on the Cotswold Line without another substantial investment in extra infrastructure.
Taunton and Exeter journey times to and from London are comparable with those for London-Cotswolds/Worcester. The faster London-Plymouth trains have much the same journey times as London-Hereford. Maybe we should start demanding the return of restaurant cars and low-density coaches on the Cathedrals Express? Though fitting in a silver-service breakfast between Oxford and London might be a bit of a push... (joke)