There ans.re a few things about this though:
1) I think the impression that has been given over the years is that the daytime services from London to Cardiff are marginal in their contribution to GWR's takings.
They are. The Cardiff staters/terminators are noticably quieter at all times of the day in both directions.
2) Would specifically an extension of GWR services to Swansea for 2tph actually be an improvement in connectivity to London or an improvement in the number of services through South Wales? If the primary benefit is the latter and the Welsh Assembly are paying, it is better done by TfW's own operator running more frequent services within Wales and London to Cardiff to stay as it is.
There's an argument to it being a bit of both. But in terms of South Wales connectivity, the second Cardiff-Swansea path would be better used by a regular hourly Swanline service, or second hourly service to Bridgend/Maesteg
3) The non-primarily abstractive test being passed appears to translate to some people travelling solely because the OA operator isn't GWR.
This is a good point and a I think a lot was made of Severn Tunnel Jcn. I think STJ has a market for services to London, but only one or two in the morning peak on the UP and evening peak on the Down. Although as I said 18months ago on this thread, it does serve a large area that use Newport for GWR to London. So in a sense, STJ can be seen as net abstractive rather than net generative, as many who would use it already drive to Newport or Bristol Parkway to catch services. The latter have 4tph in a full non-covid timetable, as opposed to 6tpd. In any case, the majority of England-bound commuters at STJ travel to Filton or Bristol TM. A service to London or Bristol Parkway isn't necessarily what they;d actually want, despite what local politicians (who don't use the train) might think they want.
4) It is not clear that the network benefit of extending a few GWR extra GWR trains through to Swansea would actually resonate with the travelling public because at (London) peak times, both weekday and weekend, GWR already operates 2tph through to Swansea.
I think there would be benefit to more London-Swansea services with more extensions in to West Wales. Whether it's actually worth doing is another question, and on balance I still think that the path could be better used by a Swanline or Maesteg service.
There was that big uproar some years ago when FGW pulled out of running the a through service to Swansea which hit Cardiff at peak time and led to crowding on the 1721 to Maesteg. This saved FGW some enormous sum in leasing costs as the train set could be used for something else. If the OA had hit the Cardiff peak, you could see that part of their proposal being revenue generative.
I remember this happening at the time and for me it has only a pain on the odd occaision I wanted a train back from London to Bridgend that passes through Cardiff needing a peak time change. Easily negated though by changing at Newport. The only people it forced on to the 1721 (which is indeed overcrowded) were passengers for Bridgend. That being the only common station. Hard to justify it for the inconvenience of a relatively small, nichce number of passengers.
There is already a current speculative thread about improving services in West Wales
https://www.railforums.co.uk/threads/speculation-on-better-services-for-pembrokeshire.213955/ and previous threads about the Cardiff to Swansea corridor for speculation - eg
https://www.railforums.co.uk/thread...wansea-bristol-taunton-services.194745/page-2. Swansea also frequently comes up as somewhere XC trains should go but again it is not their place to cater for internal demand. However, speculation on how to best service South Wales in the light of the Grand Union decision is not for this thread.
I have said for a long time that a Swansea-NE.England service would do well. Often see a large number of people changing off S.Wales services on to XC services to the North. I can't see a reason for GWR to extend to Pembroke or Fishguard beyond being boat trains, and the numbers of foot passengers is dramatically less than even 10 years ago. But yes, not for discussion here.