I pointed out that it was only 1 hr 26 from Carlisle - Edinburgh via Carstairs, and that a route via Hawick could never compete. The train takes an hour to Tweedbank as it is! The variation in distance only tells part of the story, as the route via Carstairs has high linespeeds.
Yes, if the route existed, its use as a diversionary route would be very useful. But it wont pay for such a route to be built (I estimate Hawick-Carlisle would be at least £400m, as I reckon an extension to Hawick is reasonable.
I generally find Nigel Harris to be fairly clued up and sensible, despite what some here might say. So I thought his views were out of character!