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In a turbojet, slowing down the exhaust (by adding more turbine stages) defeats the purpose of a jet engine.
And if you mean to use all the core's energy to run the fan in a turbofan, the outer bypass flow will diverge into the now low-pressure zone, causing drag, and might even cause flow reversal, i.e., air will be ingested from the back—causing a surge.
Also—at max power—a fan in a turbofan already runs at its most efficient speed, any faster and the gains will turn into losses.