It is very true that to reach the aircraft's operating ceiling you need to go to the best power mixture i.e. about 80-100 F ROP.
In the TB20 (IO540-C4) I have to do this to get above about FL170 in ISA conditions. Then one can reach FL200.
Otherwise Englishal is right - only a moron is going to fly at a few thousand feet at 100F ROP at say 65% power when they can fly at peak EGT and be burning some 10% less juice. But imagine yourself flying some old heap with no instruments... that is still the bulk of today's GA.
BTW, I don't believe LOP gives you more
MPG, compared to peak EGT, other than as a result of flying slower
I did some tests, at a constant IAS, RPM, etc and any difference was below the measurement resolution which was about 1%.