This "if everyone did x" is always a fiction, since it is impossible to get everyone to do the same thing at the same time. It is I believe a useful thought exp

This "if everyone did x" is always a fiction, since it is impossible to get everyone to do the same thing at the same time. It is I believe a useful thought experiment, for the reasons you mention. What if someone had the potential but not the determination to follow through. Or they did not have a way to train, or got injured the day they had the first competition, or the first training, or they did not like the coach or the teacher, or they preferred dancing or music instead of running, etc.