There is, of course, likewise nothing stopping* GWR running these token Stratford via Banbury services you insist it couldn't because it had franchise extensions rather than a new tender. Chiltern, of course, not even having the luxury of the DfT looking at the franchise and then amending and extending the current contract like with GWR - Chiltern having being given 20 years in 2000, that began on 3 March 2002 and will end 11 December 2021...
GWR could just have agree it with the DfT and put it in a glossy brochure like
this one from 2015 that talks about all the various benefits of the franchise extension (because the extension had improvements to train service and stock in it due to the service spec having been redone). Or even not do it with a franchise extension - eg Virgin serving Wrexham from 2008 (6 years after the franchise began, 4 years before it was due to end), or Shrewsbury and Blackpool North: which was rejected in July 2013 (the franchise extension having been awarded in March), but approved in November 2013 and started in December 2014 (though this change might have been in the extension, subject to ORR approval, but then that's an example of exactly what you wanted an example of).
Even Chiltern, without any major franchise reviews (extensions/re-tenders), have had the terms of their Stratford services changed a couple of times - and even under COVID-19 are running 9tpd between Leamington and Stratford, when the highest number I could find in their amended franchise agreement is 7 (there's winter and summer numbers, and amendments and it's a big load of legalise). Through services are still the winter 3tpd, but it's COVID so there's not many tourists and the messaging about using trains for that timetable (3rd of July) was still a very cautious one. But surely you like that it's just a token handful of through trains: that's what NSE provided, and what you want from the GWR service!
It seems that your issue is that these Stratford services aren't crack expresses (and they stop even more than normal right now!), and that it's Chiltern - a company you clearly despise for more reasons than that they don't provide a Stratford service that meets your expectations (eg your dig at Bicester Village). I get you - I share your frustration that Chiltern can and should be doing better. But I can see the light at the end of the tunnel - the franchise agreement finally ends next year.
I definitely think there's room for improvement, but the potential is there to improve with the end of that ancient franchise agreement. The new one could have the TOC buy new stock, allowing the semi-fasts not be run by 75mph 30-year old trains, as well as enabling all the Stratford trains to run through to London, as a quick fix. With medium-term improvements/changes (branch to OOC allowing more trains, post-HS2 timetable meaning Birmingham trains don't have to be as fast, signalling/passing loops allowing more trains - including on the Stratford branch itself) to enable fewer stops services to Stratford.
*Other than physical limitations of track and stock, of course. This being the real problem.
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I'm all for GWR using a reopened Honeybourne-Stratford route if they can make a business case - it's pretty direct from Oxford and there's track capacity between Oxford and the junction for Stratford - unlike via Banbury.