I read it all the way through and I think this is garbage. At least you are a HS kid who knows how to write, spell and use punctuation, but you are off the mark.
You write this:
"Most kids in high school improve a TON their freshman year. It’s common to drop 2-4 minutes from the first race to the last one. The same is true for sophomores. Coaches see these breakthroughs on a regular basis and strongly believe in their athletes’ ability to improve and shatter their PRs like they have done in the past."
So I will take you at your word that you meant this and and allow a 3:00 improvement (middle of what you stated) the first two years (you said it was common). That would mean a "slow" male would start at 22:00 and finish at 19:00. Assuming said male improves not at all over the 9 months, he starts the season at 19:00 and finishes at 16:00.
You see where the problem is already don't you?
Then you write:
"On these boards we see tons of hopeful young kids saying they want to run a 16:00 5k despite a 5:30 mile time and kids who haven’t even made it to state saying they want to win NXN. "
I myself DO NOT SEE ANY 5:30 milers with such unrealistic goals, but I will say that I do not read all the posts, far from it. But who has a 5:30 mile PR and posts things? Much less "tons of hopeful young kids"? Anyone with a PR above 5:00 usually asks for advice on how to get under 18:00 or how to run 5:00 / 10:40 next year. You are wrong on this one as well.
Using Coburn as an example of lowering your expectations is ridiculous. The only thing you and the kids you are talking about have in common with Coburn is you both have small chests and own running shoes. Not any kind of evidence for you.
Then you write:
"I remember my coach saying that every athlete on the team should be able to cut 45 seconds off their 5k PR each season. This seems reasonable for most people, but it’s not always a realistic goal. "
How so? In the SAME POST you said it was common for kids to drop 2-4 MINUTES in 9th grade and 10th grade. I am going to assume you know what the word "common" means? Again, this would mean that the average runner would drop 6:00 from beginning of 9th grade to the end of 10th grade XC season.
FWIW, I think that your coach is RIGHT, and he was probably generalizing because you are such a make-a-big-deal out of everything whiny baby. If you are well-trained you should be able to improve :45-1:15 over the course of the season in 9th and 10th, somewhat less in 11th, and only 20-30 seconds over 12th grade ... but this is for people training to their potential.
IF you are running less than 40-50-60-70 respectively, then you should be able to improve EVEN MORE each season because you have so much more to improve.