Great Western Coffee Shop

Just been working on something and an hourly Oxford - Bristol service would require 3 trains.

3 Adelantes perhaps?  ;)

I doubt the profit levels of a potential Oxford to Bristol would compensate for the high leasing costs of the Adelante fleet.

Agreed.  Sadly - very sadly - I'm sure that the commercial situation is that it's far better for the bottom line to offer travellers from Swindon and West a service that uses existing trains and requires a change at Didcot.  You'll get some complaints from those who remember the old service, you'll loose some passengers to road transport - but that's small fry compared to the extra income generated by more fully filling the "toastracks" from Bristol up to Didcot. 

Remember that the Bristol - Paddington service is busier at the London end of the route, so that runs full from London to Reading and Didcot, where a lot of people get off ... then fills up again with people headed for further west ... is an accountant's dream.  And the only section of the route where the number of service provided over and above any DfT rules actually matters ... is the short stretch from Bath to Bristol, where income has to be split with SWT based on service numbers.

The advantages of an Oxford - Bristol are very negligible compared to approximate 15 minute connections at Didcot Parkway. You would have to wait for a path onto the Oxford route at Foxhall Jn as it stands generally anyway due to the frequency of services.

Besides, if all trains are pathed correctly, the trains could essentially just have a cross platform interchange from platform 2 to 3 at Didcot.