Everything is relative. What might be a 'very popular' tourist destination in the context of the Borders (I lived there for a while and never saw much evidence of what I'd consider 'very' popularity) doesn't necessarily mean the numbers are remotely similar to those of a 'very popular' tourist destination in the Cotswolds, for example.
Unless you live there, the harsh truth about the Borders is that people tend to either go round it (via the A1 or A74) or straight through it on the A7 or the A68. Not many people stop. It's a shame in a way, as it's not especially unattractive. But, having lived there, I can see why. It's an, um, strange part of the world.
At a pinch - and it is a real pinch - if the SNP still has any money after the A9 dualling (providing they've not bankrupted us all with rising taxes, not to mention the impact of Brexit), then I can see Hawick being an attractive proposition as The Next Big Look At What We're Doing For Scotland Transport Project.
But all the rose-tinted spotter dreams (not you, Journeyman) of Hawick-Carlisle and St Boswells-Tweedmouth are utter fantasy. I did have to smile at the quote of Langholm's population of 2300 apparently justifying the huge cost of reinstating a railway, with regular users undoubtedly being a single figure percentage. Sometimes I honestly wonder how much of a grip on reality the sort of people who seem to believe that Building Railways Is The Answer To Everything have
Now now, I'm only stating that in the unlikely event of Hawick Carlisle reopening, where the intermediary stations serve 3000 people, the Langholm branch would add notably more people served for not much extra mileage in the grand scheme of the project. Anyway, clearly that won't happen.