There should be a contingency plan in place which allows for some coverage north of Preston though this might indeed impact on the southbound service.I agree - but you have to resource it, and I suspect running a Carlisle shuttle with Preston crews at no notice would require cancelling most of their London turns, which would mess up rather more passengers.
It ought to be done if it can be planned and resourced, but I am not an insider nor even a diagrammer so have no idea if that is a realistic possibility.
It might also require a degree of management-staff flexibility that would once have been taken for granted but which may not be so readily given given the current state of industrial relations.