... son has had much the same experience... HC from JC left after his second year. HC at his D2 was in his first year. HC at his NAIA has been there a long time but broke off communications at one point during the recruiting process because he was close to making a move. And the HC's are the most stable of the bunch - they at least usually have something resembling a salary. Unless you are at a well funded D1 or private, the RC's and other assistants make very little and are always on the move to try to get closer to that HC spot so they can get paid.
Looking at the conferences he has played in, I'd say 2/3 of the HC's are long-timers. It seems like support staff are either older guys with local roots who are winding down, alumni they can maybe get one or two more years out of or they are guys on the move.
This is son's first year on the coaching side. There have already been some real eye-opening experiences for something we both thought we knew a lot more about. But that's for another thread.