I don't understand why people are so surprised here. At the end of his HS career, Hunter had to make a choice - either accept a lot of money, chasing his dream or go to college, knowing that he probably will never win any NCAA title, live a poor life and be offered maybe 1/10 of what he got in his post-HS deal. For me, that's an easy choice - take the money and live the dream life.
One reason he was so good in HS was that he received very good training compared to the rest of the high schoolers. Most HS coaches are not very good coaches, think we can all agree on that. The good training he received from Tinman (and his parents, but which were also influenced by Tinman) allowed him to be better in HS than similarly talented runners. He probably never had the talent to become an Olympic champion, it's the training that made him so good.
Often people compared him to Grant Fisher here, who is roughly similar age and Hunter beat quite a few times during HS. What people fail to understand is that Fisher ran a MAXIMUM of 50 mpw in HS, and his coach assigned him no speed work until the track season of his senior year in HS. They stated this in multiple interviews. Meanwhile, Hunter did tons of CV and threshold training, which are highly effective, on 70-75 mpw as a senior. In HS, they ran similar times with the respective types of training. It should be a no-brainer to guess who will be better down the road, regardless of what career (early pro, college, etc.) being chosen.
No, Hunter would not be a 3:30 runner now had he chosen NCAA. And Fisher wouldn't have stagnated would he have gone pro after HS. Each runners development is individual, Hunter was likely overvalued by sponsors after HS because they didn't understand that his training was just better than other similarly talented HS kids, and he was never the ultra super talent that has an actual shot at winning Olympic medals.
Also, Hunter seems to be extremely prone to injury. He was never injured in HS, because the training load was not at an elite level yet. He is certainly very talented, but injury-resistance is just as big of importance. Think about Solinsky, maybe he didn't have the fastest 1500/Mile times in HS, but he could pound 100+ mile weeks at sub 6 pace year after year. Hunters training load increased after HS, and it seems his breaking point is very low compared to other elite runners.