Not having a dig at anyone on the thread, just a general point that there's a temptation to look at "now" and think that things have never been this bad/low (at any TOC).
I think it would be accurate to say that reliability and punctuality have never been as bad/low on the Cotswold Line as they are now.
RecentTrainTimes gives a peek into it, but a better insight is offered by the
27-page thread on the GW Coffee Shop which chronicles the cancellations and substitutions day in, day out, since the start of the year.
It's not uncommon for one 8-coach HST to be cancelled and the following 8-coach HST to be replaced with a 2 or 3-coach Turbo. There have been numerous occasions at weekends where a string of cancellations have left 3- or 4-hour service gaps.
I'd estimate that I currently use GWR half as much as I did a few years ago, perhaps less. Two other factors: the effective fare hike caused by introducing new restrictions in the evening peak (in 2015), and the pitiful provision for bikes on the new IETs. These were built with four 'bike and bulk' spaces, but GWR are only allowing two bikes to be carried per train.
Oxford Parkway is doing very nicely out of former Cotswold Line passengers. I'd always expected the new Chiltern service to take a chunk out of GWR's custom in Woodstock and thereabouts, but what's startling is the number of Charlbury people who are now choosing to drive to Parkway in preference to catching the train from their own town. GWR, to put not too fine a point on it, have royally screwed up.