I coached HS and college. You base your recruiting on the person and their times. Yes, the bust happens and sometimes you don't see it coming. Some kids are big fish in a small pond and then they arrive in college and see that there are 5 maybe 6 runners with faster times than them, and one or two runners from their in-coming class that are just as fast as them.
The ones that stick it out, get faster if they stay healthy. The others either suddenly discover girls and beer, or can't handle the practices and course-load combined or they just miss mommy.
I always used to say to myself if I got three out of every five coming in, I will have reached my goal. Some years, I got four out of five, one year, only one out of six remained after cross country. It can feel like a crap shoot.
Bigger programs have bigger numbers and they really don't feel it when one or two leave.
What I have found recently is that the ones that quit have told me they regret it..now.
Too late.