Frankly that's nonsense. The route wouldn't have been built in the first place were it not competitive with the coastal one
Lots of Victorian "entrepreneurs" speculated large sums of money during the "bubble" years of railway expansion - the mess that BR inherited included lots of these attempts at competition (although a number of routes were obviously closed during the "big four" era, however much people want to suggest that it was only Beeching/Marples who closed lines)...
... it doesn't mean that we have to preserve/re-open all of them, as a monument to the money that some investors "spaffed" in the nineteenth century (to use a political term)
They tried, they failed, get over it.
If you had a large sum of money to spend and it had to be spent in the south west, and you'd already at least considered electrification of the existing main lines and sorting out the perennial Cowley flooding issue and improved line speeds and refurbished stations and opened new stations (or re-opened some old ones) and were looking at what else you could do with the money then a short simple slow siding from Bere Alston to Tavistock would be worth considering, but you'd seemingly can't accept anything short of "a double track Tavistock - Okehampton line that can provide competitive Plymouth - Exeter journey times and route all InterCity services that way on the rare occasions that Dawlish is shut"...
...even though, as has been pointed out, the "successes" that people always cite as good re-openings are fairly short simple slow sidings (Ebbw Vale, Alloa, Tweedbank) and some of the underperforming re-openings are wider projects (Airdrie - Bathgate, Nottingham - Worksop)...
...look at what works, look at what can be packaged up to get Government approval, forget about the grandiose schemes for a "BML2" or the weird SELRAP idea of a half hourly Leeds - Skipton - Colne - Manchester Airport service - if you must obsess about Dartmoor then focus attentions on just the Tavistock - Bere Alston section - that's the "low hanging fruit", that might be affordable, that's the section of line you could build to compete with a frequent commercial bus service, that's the kind of package that people in the South West could argue for as payback for national funds being used on HS2/ Crossrail and other projects that come nowhere near the South West...
...insisting on the full "billion pounds" mega-project just makes it easy for everyone else to ignore Tavistock though
That's meaningless - you would need to compare the fastest possible journey times to get the capability of the route.
A non-stop Plymouth - Exeter service wouldn't be much use via Okehampton though - I thought that you wanted to reconnect all of the intermediate villages? Because you can't have non-stop services and Local Trains For Local People, unless the plan is to open a
four tracked line? Oh, wait, maybe that
is what you want?