I absolutely agree that there shouldn't be some sort of "magic number" in anyone's heads as to what a good amount of mileage is for one person. Again, as I've been saying, not everyone is going to be able to handle 100 mpw, especially when still in high school and still growing. I attributed a number to "high mileage" because that is what this thread has been talking about, wanting kids to run 100 mpw or close to it.
And yes, it is a common belief as you say, that lots of miles leads to injuries. But what I was talking about is more mileage than you are ready for. Not everyone is going to be physically ready to run 100 miles a week, or close to it, in high school. Because of that, it would NOT be better for them to run that mileage when they have found a way to succeed off of 50-70 miles.
And yes, running more miles just for the sake of running more miles, when your body is telling you that that isn't a good thing to do, IS a stupid thing to do. That is your body telling you that there is a good chance you're going to get injured if you keep increasing the volume or intensity at which you are running. Again, I don't know what's so hard to understand about that... you're supposed to push the envelop, not speed past it! Run the "high" mileage when you are physically ready for it... there is no need to rush your body into doing something it cannot handle. That's DETRIMENTAL to your health, and with a good chance (again, this is simply common belief, but there is nothing supporting the contrary and a decent amount, no doubt, supporting the POV) that you could get injured by doing so. That means you'll be off the trails for a week or more, meaning you lose time training. So why would you want to put yourself through that?
Any sane person wouldn't. People need to know their limits, coaches need to explore those limits and make sure not to push their athletes past. Stay healthy and train consistently, and you'll find success. Run 100+ mpw because you think that will automatically make you better and not listen to yourself, your body and your coaches advice, and there is a decent chance that you'll get injured if you do this so recklessly.
MILEAGE IS NOT ALWAYS THE ANSWER. KNOW YOUR LIMITS. IF YOU ARE BETTER OFF TRAINING AT 50-70 MILES A WEEK, THEN TRAIN AT 50-70 MILES A WEEK. RUNNING 100 MILES A WEEK WON'T MAKE YOU FASTER BY ITSELF!