I hear what you say, but why do manufacturers recommend this. Even the new ish 182 I fly the recommendation is 40 ROP. Admittedly below 75%
The manufacturer recommended settings were not as stupid
at the time they were invented as they look now.
The main reason is the shape of the power curve as you change mixture. From 150 rich to peak it is relatively flat, once you lean beyond peak it falls steeply. Think about it - rich of peak you have more fuel than you can burn with the air going into the engine, so having, say, 10% less makes little difference. At peak you have just the right amount, so leaning by 10% you lose 10% power. (The fuel flow at 100 rich is approx. 15-20% higher than at peak, BTW)
Unfortunately, not every cylinder gets the same mixture, but the air flow in the induction system and - in individually fuel injected engines - imprecision in the injector nozzles mean that it varies.
Which means that if you lean towards peak, one or two cylinders will reach it first and then go beyond peak. This means that these cylinders will develop less power than the others and the engine runs rough.
Hence "stone age leaning technique No. 1" - without EGT: lean until the engine runs rough a little, enrichen until it is smooth again, done. Good enough.
Now add an EGT - "stone age leaning technique No. 2". Unless the EGT is at the hottest cylinder (which can vary from aircraft to aircraft) if you lean to peak using it, the engine runs rough because one or two cylinders might be already beyond peak. 50 rich is a safety margin to avoid rough runing.
Makes perfect sense, right?
This is what IO means when saying
Operating at 50F ROP, or any ROP setting really, masks mismatched airflows and stops pilots moaning about vibration
These ancient tehniques are now completely out of date because of two things:
(1) Multi-cylinder engine monitors.
(2) Balanced fuel injector systems and better inductions, reducing the mixture imbalance between cylinders
These allow for more sophisticated leaning techniques. 50F rich in a balanced engine is a really stupid setting - maximum engine stress with little benefit over 100 rich and little more power than peak.
The engine manufacturers are behind the curve because these two things were done in the aftermarket - a good dose of "not invented here". Also, none of the engineers around nowadays has actually designed the basic engines flying now. Little wonder many of them have less of a clue than people who modify them to make them work properly.