What is happening with Brosnan is the logical conclusion of the improvement at higher levels (professional and college) trickling down to high school. I might be getting the specific details wrong but here's what I have observed over almost 4 decades of following track:
In the 80s, there was very little true professional support for track athletes. Many of the people trying to break through in the professional ranks had actual jobs and trained the best they could while working full time. Though there were some attempts at creating professional groups (Reebok Enclave for example), it didn't really lead to big results most likely because the money wasn't there to allow people to 100% focus on the sport. It feels like things changed around the time NOP started; then our professionals started getting some serious results and momentum started. More teams like NOP formed. Now we have a lot of pros who have great contracts and don't need to think about how to pay the bills.
A little after the pros started to train smarter and more professionally, things seem to have taken a giant leap forward in college. You didn't see a lot of college guys running under 13:30 until about the time Rupp, McDougal, and Solinksy were around, then suddenly guys were breaking 13:20. Now look at college: lots of guys approaching 13:10. Supershoes is part of it, but I think the effect (while significant) is overstated.
Since college is getting so difficult to compete in, some high school coaches have started training their athletes more professionally, realizing that young kids can do much more than we gave them credit for. Brosnan isn't the first, but seems to be the most dramatic example. He has raised the bar in high school and you'll probably see a lot more coaches investigating how to coach high school kids like pros instead of the way it was when I was around in the late 80s: "Take the winter off and rest your legs for track season" and "I only want you to run 30 miles a week so there is some upside for you in college". There was a reason people sucked so bad in the early 90s. We were losing valuable years of development that can't be made up later on. As more coaches begin training kids year round and not being afraid of them running an hour a day, if a team wants to be competitive , they'll have to up their game. It will spread and raise the level everywhere.
Look at the high school lists and imagine that coaches like Brosnan are still few and far between. It would not take a lot to see the number of Brosnans multiply by 10 over the next 10 years. I predict that within a decade we could reach a time where there are 10 high school guys under 4:00 for the mile in a given year, maybe more.
Let's watch and see what happens!