Have you ever heard of a thing called the service level commitment? This is part of every rail franchise agreement in which the Department for Transport tells the franchisee what services it is required to provide on each route.
It is not up to the operator to 'favour' one route over another, it does what it is told to do - and in the case of GWR and LNER, when it comes to the Class 800s and 801s, uses the trains it is told to use as well, on terms that were agreed by the DFT and Hitachi, not the train operators.
Unless you know different and can show us some clear evidence of this supposed favouritism, please don't post unsubstantiated claims.
Amazingly, the railways have thought of this kind of thing, which is why there are extra trains to the West Country in the summer holiday season every year (and at Easter and Christmas) and changes to timetables and rolling stock allocations for events like Bath Christmas Market.
From Somerset Live on November 26, 2019
https://www.somersetlive.co.uk/in-your-area/extra-trains-bath-christmas-market-3580657
If you want to ignore the point I have made about the relationships between the trains that go down to Penzance and back to London across the course of each day and from day to day, that's your problem. But I'm afraid there is no getting away from it.
If you want a nine-car train on a particular Penzance service out of London, then the service that set works coming back the other way later that day, or the next morning, is inevitably going to be a nine-car, whether or not it appears on your little list - surely it's not that hard to grasp such a basic point?
As a result, your little list then becomes nine-cars on rather a lot of Paddington-Penzance services - which you have been told over and over again is not affordable unless you bin the 2tph frequency on the main line in Cornwall - so which is it to be?
And do try looking on realtimetrains at the series of ecs moves from Long Rock depot to Penzance station every morning before the London departures.
The first London service that is formed by a set that comes from Plymouth is the 10.15, which you regularly demand should be a nine-car all the time. And guess what that set appears to do once it gets to Paddington? Forms the 16.04 to Penzance. Every IET sent to Penzance from the 14.04 onwards overnights at Long Rock.
Apologies, my mistake. But as you say, many people are probably still thinking old timetable, not realising that they won't get turfed out at Swindon now if they get the next train, whatever day of the week it is - and the weekend between Christmas and New Year is still not a typical one for travel patterns.
This is probably just the sort of individual service that GWR will be monitoring anyway, to help decide if they have got the allocations of stock right, or whether some fine-tuning will be needed at some point next year. Determining how many people off to London for a Saturday day out want to arrive at 10am or 11am is probably not the easiest thing to predict, especially on route with one of the biggest changes to its operations in the new timetable.
A five-car GWR IET seats 326 people (36 of those in first class), a high-density HST was something like 580 (with 76 first class seats in 1.5 coaches of first in their final configuration).