take off the bulk of the London bound passengers from an Exeter - Waterloo service and you may have to reassess things
Based on many years' observation using the Exeter-Waterloo route, the biggest markets west of Salisbury seem to be:
1. Yeovil and stations eastwards towards Waterloo.
2. Axminster and stations westwards towards Exeter.
3. Through Exeter-London journeys.
4. Exeter (and possibly stations west of it) to Southampton and Portsmouth via Salisbury.
5. Intermediate stations to Bath / Bristol or Southampton / Portsmouth via Salisbury.
However the first and second markets are much bigger than the remainder - the quietest section is generally between Axminster and Yeovil Junction. Whilst it might be faster for some folk in the Blackmore Vale to get to Castle Cary than Sherborne or Gillingham etc., the roads aren't generally that fast and the train frequency is generally 1p2h rather than hourly.
Harder to comment on other lines but a reduction in Exeter - Waterloo journeys specifically probably wouldn't make too much difference to the line's economics.
Given that we have an Okehampton station regardless, I can't see the harm in promoting it!
A key difference between Okehampton and Corrour is that the latter is over 2 hours from the Central Belt and has 3 trains a day. Not particularly conducive to encouraging hikers, particularly if the weather turns.
Okehampton will be 45-50 minutes from Exeter, and not much further from Exmouth or Newton Abbot, and will have a 2 hourly (hopefully becoming) hourly service. Rather more conducive to encouraging walkers to head over for a day out, particularly given that you can walk to the highest points of the moor at High Willhays / Yes Tor in 1-2 hours (I've done it myself using the Sunday rambler services in the past). The line is hardly likely to be overrun with ramblers for 9 months a year, but it should provide some custom to a route which is already due to reopen.
However I'm certainly not suggesting that there are hordes of ramblers at Tavistock and Plymouth just itching to catch a train to Okehampton to visit the north-western edge of Dartmoor or that this will create a compelling financial case for reopening.