If you look at the shorter of the two previous alternative routes - the Teign Valley - it was a meandering single track route, no doubt steeply graded. The Heathfield branch as exists today from Newton Abbot is itself prone to river and tidal flooding. There are two tunnels one of which (Culver) is still open but the other (Perridge) is blocked in the middle by a roof-fall and very wet (Source: Forgotten Relics - http://www.forgottenrelics.co.uk/tunnels/gallery/perridge.html ). The route via Okehampton had the advantage of being a main line, but would require two reversals (one at Exeter and again at Plymouth) on a through service from Paddington to Penzance. So while it would be nice to see the old line(s) re-opened, in cold hard economic terms it is going to be cheaper to rebuild the damaged section and fortify the defences against future storm breaches.