This posting indicates that although it is possible to travel from the South Coast conurbation to stations further west, the timing of the connections make a journey longer than might be ideal because there is only one train an hour from Portsmouth to Cardiff which can't be timed to make every possible connection along its route.Your right that the route can be covered but let's not hide the fact that it's rail pulling out from the South Coast Conurbation to West Country Market. However easy the same platform connections are at Fareham a 30 min wait in the cold in the middle of lengthy journey is not ideal. The loss of Exeter connections at Westbury from Brighton have added the best part of an hour to journeys to the Southwest. From anywhere east of Barnham the quickest way is now via London. From the South Coast to Cornwall now the obvious public transport connection is the morning flight from Gatwick to Newquay.
It is also recognised that GWR have finite resources and that running diesel trains further than necessary over electrified routes isn't ideal.
Would there be an advantage in having a regular interval 2tph service using the same resources as the current 1tph Cardiff to Portsmouth service but just running between Bristol and Southampton?
Such a change would increase capacity over this part of the route, perhaps replacing the current Salisbury to Romsey service. It would offer more connections along the route at Westbury and into services from further east and west.
Southampton has a west facing bay platform which could be dedicated to this service. Bristol would have interchange as well.
Trains at the start and end of the day might continue to go to Portsmouth to access the stabling point at Fratton.
SWR might then run Portsmouth to Bournemouth (and in time Fawley to Romsey via Eastleigh) and a different service could run into South Wales from Bristol.