Yet regularly on here people suggest the Sunday service should be the same as on a weekday!
Clearly, the suggestion is untenable on a blanket basis as many commuter type flows are just dead. However, some longer distance services from Sunday late morning onward need to be at least as or more intense than on a weekday.
Extrapolating from my own painfully acquired ample database of GWR (mostly) anecdata, it's ludicrous that there are only 2-3 trains per hour to London from Swindon (and correspondingly fewer for all points west by definition) on a Sunday afternoon versus 4-5 on weekdays. The long list of explanations for this situation are very interesting and all that, but sound like something from 1952.
One attempt at such a journey ought to be enough for anyone who isn't either a) desperate, b) a masochist or c) both. It's a deplorable experience most times. I feel sorry for all the naive first-timers who were expecting something more civilised in return for the £eye-watering outlay. Hardened regulars come psychologically prepared to figuratively punch their way onto the train and claim some barely-existent standing space to wedge themselves into. They then get to stand in an aisle/vestibule/toilet or face the wall in the cycle cubicle for a few hours. It's outside my experience, but the same seems likely true in the other direction too given that when you get to Paddington the place looks like some epic disaster has hit London and everyone is desperately trying to leave, often with all their possessions in-tow.
More limited experience of heading to London from just about every other direction at this time suggests things are much the same. E.g. spending the entire journey from Manchester to London standing wedged against a door astride someone else's suitcase because there was nowhere else left to put it that didn't involve holding it above our heads for the duration.
To cling to the thread topic, Saturdays on this route at least used to be rather civilised on the whole unless there was a relevant big football match, in which case it could be expected to be uniformly grim. Now they are fairly often a bit unpleasant, but rarely a patch on Sundays.