If you want to go on about what used to happen, that's up to you, but maybe you should come and spend a day at Oxford now.
Splitting and joining of Turbos at Oxford, ie crashing them together at low speed when coupling, without lots of technology that needs to talk between sets like IETs, has been limited since 2004, when 180s took over lots of Cotswold Line duties and the Stratford-upon-Avon service ended, and even more so since the December 2006 timetable change, to the point where it has almost been eliminated now, with the December timetable probably putting paid to it for good.
The overall number of trains - passenger and freight - through the Oxford area has increased steadily since over the past 20 years, while the number of through platforms has remained exactly the same - two. And the volume of traffic through Plymouth, with all its through platforms, in no way, shape or form, remotely resembles what is passing through the Oxford area - or shunting around the station and sidings.
Plenty of the existing Cornish services have extended dwell times at Plymouth anyway - another of the many things you gloss over in your posts, unlike at Oxford, where the priority is to get trains in, out and on their way as fast as possible, as something else is usually breathing down their neck in both directions. Looking back at Monday-Thursday last week on realtimetrains, apart from the one or two services with a long dwell and late departure from Plymouth which may have been recalcitrant IETs, every other service that left late, in both directions, had arrived late at Plymouth due to a delay somewhere else, so clearly nothing to do with coupling or uncoupling.
Maybe it was just a good week, or maybe things aren't half as bad as you keep trying to make out - with zero detail provided by you as to how often 'lots' actually amounts to.
And any delay to a service operating heading into the Thames Valley at 17.31 (which is when the 15.14 from Hereford leaves for London) is going to cause a whole lot more grief to a whole lot more services than an issue at Plymouth.
The redoubling of parts of the Cotswold Line in 2009-11 was not motivated by any great desire to increase capacity, or run more passenger services on the line. Network Rail justified it on the basis that it would improve punctuality of trains arriving at Oxford from Worcester, increasing the odds of them reaching Didcot East right time and then running in the booked path to London, thus reducing the risk of Cotswold services wrecking the timetable into London. And back out as a result of late arrivals at Paddington. Which did indeed happen 'a lot' and was why £67m was spent addressing the issue.
