A difficulty with building an entirely new route is that the Exeter-Torbay corridor is a populous one by Devon standards and therefore a relatively important one to serve by rail, but moving all Cornish traffic onto a new route would make the economics of the current route weaker. That said, as NR are responsible for maintaining the sea wall regardless, I understand they've taken the view they might as well allow trains to run along it.
Contrary to popular belief (although it hasn't yet appeared on this thread), building a new route between Exeter and Newton Abbot would only save a handful of minutes. Voyagers can currently do the 20 miles between them in 18 minutes. The slow section is the 32 miles between Newton Abbot and Plymouth, where non-stop services can't average 60mph.
The long-term options do appear to be: 1: Carry on as now for as long as is physically viable. 2: Build a new line between Newton Abbot and Exeter at massive short-term expense which will save about 5 minutes at most and abandon the existing route. 3: Build an entirely new route between Exeter and Plymouth at even greater expense and try to maintain the existing route as long as possible, or build a spur off the new route to join at Newton Abbot, which will also be expensive.
Much as I'd like to see the old route via Meldon open in full, it isn't going to happen because it goes through a much more sparsely populated area than the current route.
The Teign Valley route has less chance of reopening than a high speed link to Atlantis. It was a slow meandering single track country branch line with almost no passenger traffic which closed in 1958, several years before Beeching, and the half mile long Perridge Tunnel has collapsed anyway. The cost of trying to realign a Victorian era branch line to high speed standards with double track clearances is probably more than building a new route.
