40-50 MAX?
I won't get started arguing over that, as that is asinine. However, I will tell you something you may not have thought of. The U.S. and Australia both produce outstanding swimmers, in all events. So they both must know what they are doing (according to the logic you used with distance running). Most people know that it is much harder to swim a mile than to run one. High school swimmers swim much more than the running mileage you propose is ideal, the elite ones are doing that much yardage by early teens.
Wouldn't this give them the same neurological problems, immune system problems, and growth-stunting, etc.?
To cover a mile in swimming takes at least 20 minutes since it is so much slower, there are breaks, etc. So to do just 10,000 yds a day takes at least 2 hours and more like three.
My biggest problem with what you propose is that all it takes to show yourself you are wrong is to try it yourself. 40 miles a week by 18? No decent runner (4:30-1600, 9:45-3200) is going to be pushed at this level, they won't even feel strain. I agree that only those sho can tolerate it should PUSH THEMSELVES to run 80-100 mpw. Most US coaches only ask kids to run 35 miles during the week and don't even have AM runs.
But seriously, 40-50 a week by age 18? It looks like this:
M: 5 easy, 10x100m
T: 1 WU, 1600 race, 800 race, 1 WD
W: 6 easy
T: 1 WU, 6 x 800, 1 WD
F: 5 easy
S: 1 WU, 1600 race, 800 race, 1 WD
Su: 7 easy
This is 40 per week, which would be fine for a freshman, or a 9-10th grade female, but a senior is very close to as mature as he is going to get. A schedule like this is cheating the runner out of reaching for his potential. And where are the two long runs you speak of? The longest continuous run would be 45 minutes?
I think you are interpreting what I wrote as "every HS kid should run 100mpw year-round"... that isn't it. Very few should do it and it should be confined to about 30-40 weeks of the year, but certainly some should try it.
You keep saying that the US has never produced a "distance champion", but how many Australian champions have been produced from waiting until 23 to run a little bit?