Tired of this argument. Some don't improve in college. Some actually get worse. Some HS kids have already running been running high mileage. You have literally no control group here. You would need to find the times of HS kids who don't run in college but still keep at 40mpw to see their improvement and accurately compare.
Better evidence would be, what's the biggest improvement someone has gained after plateauing at 40mpw for at least a year then going high mileage? A long term plateau would generally mean you've reached the potential for that mileage or that training plan. Let's hear a few stories on that and then maybe you can get a better picture of what the real gains are.