It's debatable whether the ferries actually provided much of business case for keeping the line open in recent years.
My view is the strongest business case for the line is local passengers plus ferry passengers. I was on the ferry 11.30 ex Cairnryan on Friday there were 250 foot passengers, most came by bus. I checked seat availability from Glasgow on the coach the previous Wed for Friday and it was already full. With a decent train service from Glasgow and a convenient transfer to Cairnryan by bus (journey 10 minutes) many could have travelled by train. Translink (Northern Ireland) seem only interested in their bus service, and Scotrail & Stena/p&o seem not to be able to work together effectively. Looks like there is little hope for this line unless someone can get the rail sea organisations to work together. So much for Nationalisation.
Pricing is a key issue: a return day rider ticket to Glasgow and area from Girvan or Barrhill is £10.70 (1 adult min) plus add on £13.60 Stranraer to Barrhill. Or £26.90 is the off peak day return to Glasgow. Compare this to Southern's £10 off peak dayrider if it was £13.60 with an add on £5 it would generate lots of new business.
Finally I checked the train at Ayr to Stranraer before it left on Fri there were approx 10 passengers, probably most for Girvan