You make some good points, but in practice there are issues. It is true that there could be a low VO2MAX 100 mpw junior with a 5k of 17:00, and then another with a high VO2MAX, same 100 mpw and running 14:30. They both can handle 100 mpw equally well (just takes a bit longer for the 17 min guy).
They do the same workouts, like 8-10x1000, one at 3:30 and one at 3:00. Or 20x400, again at speeds similar to their skill level. Where is the problem now? Recovery rate. The high VO2MAX guy can recover much faster from workouts and intensity than the low VO2MAX guy. The problem is that the HS coach probably assigns them the same volume in workouts, so let's say both do 9x1k at their prescribed pace. The 14:30 guy can handle the workout well and recovers fast, but the low VO2MAX guy is being run into the ground with these sessions. He doesn't recover fast enough during the workout, after the workout, can't clear lactate fast enough etc.
There are many studies, like
https://www.ncbi.nlm.nih.gov/pubmed/11219498that tested this relationship. Having more/bigger mitochondria, increased concentrations of aerobic enzymes, increased myoglobin, blood volume and hemoglobin all accelerate the recovery process and oxygen use for a high VO2MAX person, even if paces are set to current fitness level and total mileage is the same.
It's not easy to individualize training for each HS athlete, some respond better to high mileage (often ST-runners), some to intensity (FT), but most coaches don't have the level of experience to know when someone can handle high mileage and benefit from it and when not. So I'd rather have them be cautious than automatically assign 70 mpw to all girls and 90 mpw to all boys, I'd rather have them prescribe moderate training loads and only if someone is really ready for high mileage prescribe it for that individual.
Did Grant Fisher run high mileage? Did Drew Hunter? No, they had great coaches to develop them which allowed them to gradually increase the training load and reach new levels in post-HS. It's more important to keep athletes healthy and allow their talent to shine than to go for quick improvement and risk injuries and burn-out.