One more thing. Your last paragraph was good. Here's the way I see it. Some people, due to slow/fast twitch ratios, how efficient their anaerobic vs aerobic energy systems are, and other physiological details, are better at certain events compared to others. Ther are mental factors too. Can you more easily tolerate an hour of discomfort or a minute of greater discomfort? What kind of training are you more accepting of and are therefore willing to do more of and do harder? OK, we all know this; so what?
Those who are best suited to 1500-5000m are served very well by what you (and every high school and college coach) are suggesting. Many greats fit the genetic profile I refer to. El G, Lagat, Jenny Simpson. There are also various high school prodigies who do. Who will be on the US 2024 marathon team? I can tell you who the 3 won't be: Cain, Ephraimson, and Cranny. Given their physiology, they ran 'approved' (again, by the court of public opinion) events at 17 and succeeded. To pressure or force a teenager whose genetics make them potentially better at something longer to run mid-distance events is only going to discourage the kid.
I look at myself. Modest though my PRs are, I could tell early on that something like the mile wasn't any ticket to glory for me. I was much better at 10k. Much better, but perhaps not Olympic caliber. OK, who among us is? Thing is, winning my age division in a bunch of road races was exactly the encouragement I needed as a young, developing athlete. Like nearly all of us, it didn't turn into a six-figure cash cow, but I'm doing it in middle-age and can still pick up an age-group medal. So I'm not incredibly bad at it - if I'm allowed to run events longer that half an hour. If I had been stuck at 5k and down, I would have been dead last at a bunch of mile races and never tasted success (modest though my success was). 10k and up was considered OK at the time, fortunately. Parents, peers, everybody approved. I was decent at it and it was cool. Let me reiterate: I am happy to see that things are moving back in that direction now.
Here's where your last paragraph comes in:
We just don't know how the young lady I mentioned would have done at something like 1500 or mile if the forces aligned for that to be her primary focus. Almost certainly not nearly as good as she was at the marathon. How about our current heroine, the subject of this thread? Would she have won national HS miles and such? Let's say that short stuff was all she was aware of and there was no knowledge of marathons or even halfs? Would she be the best teenager in the country? How about the best all-time? Definitely not. Let her run her best events.