EPR is just a calculated controlling parameter
N1 is just a calculated controlling parameter
IEPR is just a calculated controlling parameter
The engine generates thrust and needs to be protected, it needs to be kept within limitations. As mentioned before, the limitations change and the highest possible generated thrust changes as well (temperature and pressure depending).
EPR, IEPR and N1 values always need to be corrected for current atmospheric conditions, so to a pilot none of them actually make sense. One has an andvantage (don't know if it's EPR or N1, should have to go back in the books though). We might think N1 is more adapted to our pilot environment, but it isn't really. Otherwise nobody would wonder why N1 and N2 values can be greater than 100%. 100% does not mean a damn thing. I can understand a pilot says "EPR doesn't mean a thing anymore". But so does N1. Say we need full takeoff power. Today that might be 95% N1, tomorrow it can be 105%.
EPR is still perfectly usuable for fan jet engines. On low bypass ratio engines the parameter might be more understandable for us, pilots, yes. But that has never been the goal :-).
BTW, even fuel flow can be used. Never flown the 737-200 with the JT8D in real, but in the simulator IAS/2 was your fuel flow. 250kts? 1250kg/h :-).