It could be but as far as I can see the whole thing very much feels like an answer looking for a question. I’m all for through journeys to slightly smaller places but not at the expense of everyone else’s punctuality.
Trains extended to Okehampton would lay over for a similar time (which is practicable with one every two or three hours).
Barnstaple hasn't got through trains to Waterloo because the hourly service means the train has to be in and out within 5 minutes and punctual working is maintained by a long layover at Exmouth Junction.
SWR/GWR joint working solves the crew problem as GWR crews take over at Exeter (and stops it turning into an Orcats raid)
It is most certainly not an answer looking for a question though. A huge problem is peak hour paths between Exeter St Davids and Exmouth Junction. A shuttle from Okehampton to St Davids would be poorly used with pax facing a mile long walk up the hill to the city centre or changing (with risk of missed connection to Oke on way home). Through running to Exeter Central is essential if the service is to succeed.
Extending the Waterloo trains to Okehampton solves this and gives the additional benefit of through Okehampton to London trains, which to be honest is needed if Okehampton is to develop as a viable railhead/parkway in the longer term. Not much point driving to Okehampton if you have to stand on a cold platform at St Davids waiting for a late running London train or miss your train to Okehampton due to late running from Padd and have to spend a fortune on a taxi. Might as well stay on the A30 to Exeter or Tiverton Parkway. Through London train changes that.
Prior to WR taking over the line, the SR "Beeching" plan was to retain as far as Okehampton and Barnstaple; with all stations west of Basingstoke shutting except Andover, Salisbury, Gillingham, Sherborne, Yeovil Jct, Crewkerne , Axminster, Honiton, Both Exeters, Crediton, Eggesford, Barnstaple and Okehampton. Full width "Hastings" DEMUs were to run intercity services to Okehampton and Barnstaple, which would serve as parkways and bus interchanges for onward travel (with a St Leonards type diesel depot at Exmouth Jct to house them and another thumper build for any east Devon services that survived Beeching).
That is why those stations were retained in the Beeching report.