I've never visited Stranraer myself, however it does appear to be quite a emote settlement in the UK context.
Perhaps the railway could be better timed to serve the population.
I agree that NR are public servants and it's not for them to pick and choose which routes they deign to maintain.
The Stranraer line is an example of where a set of interacting changes over decades have created a stupid situation.
Pre-Beeching, there were two railways to Stranraer. The "Port Road" from Dumfries, and the line from Girvan. Both go through the back end of beyond, with little intermediate local traffic.
Both were put up for closure, and the end decision was to reatin the line from Ayr because it would maintain the connection from Glasgow, and the traffic for the ferry could be diverted (45 miles further) via Ayr.
More recently, the ferries have been moved from Stranraer to Cairnryan and the connecting coaches run from Ayr because it's more direct (15 miles shorter distance than the train to Stranraer), slightly quicker, and can connect with the half hourly Scotrail service from Glasgow, rather than a boat train timetable. The rail service has now largely been curtailed to run to/from Ayr rather than through to Glasgow.
That leaves a 60 mile (40 from Girvan) branch that serves few local communities, runs through challenging country (long stretches at 1:50 - 1:60 in both direction) and delivers limited value to those communities - even back in the 1930s, Dorothy Sayers' "Five Red Herrings" had characters driving cross country rather than relying on making connections by train. Diverting it to Cairnryan would be expensive, even using the trackbed of the "rather lumpy line" of the Cairnryan military railway; serving Cairnryan (pop. 142) and Stranraer (pop. 10.5k) would be challenging.
This isn't about timetabling, but some really fundamental constraints on what you can do in remote country.
I'd be sad to see it close - it's a lovely line (and with a class 56 makes a lovely noise!), but it has much less social value than the Far North.