The original plan was something like Didcot to Maidenhead stoppers and TfL not going further west than Maidenhead but Reading has been the natural endpoint for the stopping service from London for a long time.
It is a bit unfortunate that off-peak demand at Twyford and Maidenhead has never grown to the extent that they can justify a service which crosses to the main line for a non stop run into Paddington. In the past, local connectivity was important but TfL Rail provide the connections to the intermediate stations now. However, keeping the off-peak Twyford and Maidenhead train on the relief lines has been the preference to date
Even if the off-peak demand was there, a service for Twyford and Maidenhead, then fast to Paddington on the Mains (with possibly a call at Slough) from Maidenhead East or Slough West, is no longer practical. Off-peak there are now some 10 or 11 Reading to Paddington non-stops per hour on the Up Main and, of course, a similar number on the Down Main. As the crossings from the Reliefs to the Mains are all flat, the lines being paired by use, crossing moves should be avoided if reliability is not to suffer.
This would not be so critical if the crossings were grade separated, but Didcot East needs to get sorted out first.
How about running the Elizabeth Line to Didcot, extending the Reading semi-fasts, or the Maidenheads?
Probably not going to happen but one wonders if it would have been a more sensible option
Breaking the outer-suburban Thames Valley service at Didcot was a temporary measure while the Didcot to Oxford section was electrified; it should not be made permanent as the change at Didcot has caused a drop in custom as a previous poster has indicated.
Ideally in my opinion, the Oxford 'semi-fasts' from Paddington would call at Slough, Maidenhead, Twyford, Reading and then all stations to Oxford. There may be an argument for a stop at Ealing Broadway in the peaks. As I wrote above these will have to run on the Reliefs as conflicting moves are inevitable - wherever the crossing move is made - because of the intensity of services on both Mains and Reliefs between Paddington and Reading and the lack of grade separated connections between the two.
Crossrail offers essentially an 'all-stations' service to Paddington - one or two stations are skipped but, seen from the 'country' end, it is effectively the slow train. Extending the Crossrail services to Didcot would mean a slower overall service between places like Pangbourne and London.