Takes a deep breath: can anyone explain how costs of maintenaning a fleet like that on the ground, would way up against flying empty? At what point would it make sense to "mothball" airplanes for longer term storage?
Maintenance costs are based on different schedules, there is usually a time limit (as in calendar time), a flight time limit and a cycles limit (a cycle is one take-off + landing). Which one is more limiting depends on the type of operation the plane was designed for, A long-haul plane is designed for more flight-time vs cycles, a regional jet is designed for a high cycle count. So maintenance costs will decrease if you keep them on the ground, but some components may then "time-out" which will still incur maintenance costs to bring them back to airworthy condition.