Passenger numbers are something of an irrelevance if you're putting together a business case for closing a line on a pure profit and loss analysis.
The Far North Line and the West Highland Line are so unprofitable, their closure could easily be justified on pure financial terms, regardless of the passenger numbers - they would need so many more additional passengers to make the routes viable, but you wouldn't be able to fit them on the route without significant doubling, platform extensions, and those passengers have to come from somewhere, perhaps by increasing the populations of the settlements en route.
The only ScotRail route which routinely turns a profit is the Edinburgh to Glasgow via Falkirk route, the SPT routes run around break-even and the remaining routes run at a loss, the profit from the E&G covering some of the shortfall, a healthy government subsidy making up the remainder.