I have flown with well used engines twenty five years old that were on tuned pipes and still strong enough to chew up your hand and spit out the bones.

Generally, the more modern engines are even more well made with tighter tolerances. There's no reason why an R/C engine can't last decades when used in typical sport flying applications. What is the nature of your trouble with the two AX engines? have you disassembled them to have a look? The fact that the 75AX didn't run well inverted makes me wonder about your tuning accuracy.