I completely agree with you that until recently they let them get filthy and run down. I'd rather have a clean, old monorail than a new monorail that never gets cleaned. But unless there's an unavoidable limiting factor, like metal fatigue on airplanes, there's not actually an overarching recommended replacement timeline because all of the individual components have their own replacement schedules - the tires get replaced more often than the motors, which get replaced more often than the control system, which gets replaced more often than the windows.
If the chassis is good for 50 years, which... if it's just welded metal is pretty realistic... then yeah, these refurbishments are actually enough. If cleanliness is the actual issue... which I think it is... the cabins on these refurbished trains are going to get beat up at the exact same rate a cabin on a brand new train would.