Well, parts wear out regardless. In high voltage/current and high frequency devices like monitors and power supplies, they wear out a whole lot faster because the transient currents eventually cause micro cracks/fissures in the silicon die itself, then failure occurs when it gets too bad. 

If its not continuous run then failure, its the start-up surge that kills them. 

Leaving them on is typically worse because it wears out the CRT, and the CRT for a fact isnt replaceable or obtainable. 

All the other jellybean parts minus the proprietary transformers/microcontrollers, is obtainable.