What happened to the power plants after an aircraft is retired?
They'll go on another airplane, or go into inventory as spares, or maybe go for overhaul if they are becoming or close to becoming worn out (the can be run until the Inter Turbine Temperature at takeoff or oil consumption becomes excessive, or they fail an internal inspection, or suffer excessive vibration etc).  Engines are changed all the time, every few years, and can be thought of as a separate asset from the individual aircraft.