I do think where coaches set the bar is significant to how successful a runner will be.
Many coaches look at sports as something kids are doing to check off a box and don't care about performance, only participation. Then they coach as though distance running is where all the non-athletes go and they don't want to hurt people's feelings. They try to make it some kind of non-competitive equality thing and harm the serious athletes. My high school cross country coach was like this (we had a different guy for track).
At the next level, you have the coaches who are only going to give you so much work because they have limits in their minds about what a high school athlete can/should do. They set some kind of arbitrary level as "good" because there is so much underachievement at the high school level. 4:20 has often been a number associated with being considered good. My high school track coach was like this. He was a PAC-10 school record holder in the 5k/10k, had himself run lots of miles in high school and college, and knew how to get fast. But he was holding us back to 'save our legs for college'. I ran 4:20 in HS on 6 months a year training of 30 mpw or less and felt sh!tty about it because I knew I left a lot on the table by trusting my coach.
Then you have the Brosnan types. They know that kids want to realize their potential. They don't set limits on what is possible or what a high school runner "should" be doing. They just apply training principles and let the results work themselves out. Since running is a pretty difficult sport, it takes a great motivator to get enough kids to buy into the program that you'll eventually have a bunch of stars. Pat Tyson was like that.
The "4:20 isn't that fast" thing is true. In my relatively small county, there were always a few guys around who were 4:20ish level on soccer shape or minimal training. I'm a terrible coach and I got a guy like that with 4:23/1:57 PR down to 1:53 with just a couple months of putting in 40mpw instead of 20mpw. He eventually ran 4:11 in college but was the guy who was afraid to run more than 25mpw because he might get hurt.