The thing with airlines is that their fleets include hundreds of planes and they can easily take planes out of service for maintance and overhaul. The same goes for the resort rooms. Disney has thousands of resort rooms, making it not such a big deal when they have to close parts of a resort to do renovations. The monorail system is not the same case. Disney has 12 monorails and no more. They cannot just build more monorails to solve the problems. The monorails are a necessity for transporting people to and from MK and Epcot and when not all 12 monorails are running, it becomes very crowded and there are lots of unhappy guests. If Disney closed the entire monorail system for a month or so to do a complete overhaul on the trains, Disney would receive so much sh*t from people, it would be unreal.
So, do the monorails need a lot of TLC? YES!!!! But, is it feasible to take monorails out of service to do major overhauls? NO!!!