As far as the various recent route strategies explain it,
XC▸ cannot extend the rest of the Reading terminators because there is a lack of paths south of Reading and Basingstoke in the other hours, which are used by freight services in one direction or the other. They seem quite clear that an additional unit wouldn't allow the service to run, it needs additional infrastructure.
I'll try and dig out the relevant explanation tomorrow...
Here goes, it is from the 2010
GW▸ RUS▸ :
To improve connectivity, and assist with capacity issues to the South Coast, the RUS reviewed the option of extending the current Newcastle to Reading service to Southampton and/or Bournemouth. A high level economic appraisal proved that extending to Southampton provided sufficient value for money for further consideration. Timetable analysis was undertaken on an hourly extension and a two-hourly option.
The hourly option was discounted due to the significant amount of infrastructure that would be required. The two-hourly option, providing an additional six trains per day in each direction, between Reading and Southampton proved feasible on the current infrastructure and was assessed against the proposed freight growth as per the SFN forecasts to 2019 and 2030.
The analysis proved that the extension of the passenger service on a two-hourly basis would not compromise predicted future freight growth to 2030 and that all services could be accommodated on the existing infrastructure. The RUS therefore recommends this option subject to performance modelling of the proposed service extensions in the Basingstoke station area...
http://www.networkrail.co.uk/browse%20documents/rus%20documents/route%20utilisation%20strategies/great%20western/great%20western%20rus.pdf AFAICT▸ , it's the more recent Wessex route study that now hints at the infrastructure changes required; such items as grade separation at Basingstoke, and additional or lengthened freight loops through the Micheldever and Wallers Ash areas.
Paul