What Notso Fantastic may have been trying to say is that N1 is just a percentage of rotational speed. EPR is the ratio of inlet to outlet pressure. As both engines get older the N1 percentage remains the same but the output of the engine deteriorates. This isn’t presented to the crew. An engine that measures its performance with EPR won’t be able to deliver the same performance as a new engine. The EPR gauges will show this to the crew as a lower EPR value.
Well this is not necessarily true and while your description of how things work is OK the judgements of cause vs effect is not.
In both cases(N1 vs EPR display in the cockpit) it all depends on how the engine is controlled and what FCOM procedures are trained.
For the high bypass engines the majority of the propelsive force comes out the fan duct, but is rarely displayed as such to the pilot or controlled by the engine control system. The displays to the pilot as well as the contolling effects of the engine vs PLA in the cockpit are mostly synthesized effects based on assumed engine health within limit ranges. Inh other words don't wrack your brains about it.
In the event of damaged engines, i.e. FOD, this synthesization breaks down and the display in the cockpit of either EPR (out the jet exhaust) or FAN N1
do not reflect actual propelsive thrust (it will be less) Under this condition it's best to just fly the aircraft , keeping EGT within limits.