There are a lot of people who go on training jiu jitsu (or other combat sports) for many years and they remain as uncompetitive as they were in the beginning. S

There are a lot of people who go on training jiu jitsu (or other combat sports) for many years and they remain as uncompetitive as they were in the beginning. Some because of athletic limitations, some because they are unable to put into action what they have in mind, and some because they lack sufficient intelligence. "Study or practice for enough years and you will get good enough/competent" at the population level sounds good, but it is simply not true.