[DC 86]

I also think there are very few "career" OL and DL coaches in the college ranks. Most guys in that job expect to become coordinators in a "few years".

This is incorrect. OL and DL coaches are by far the most likely to coach those units for a long time without ascending to coordinator. Because of the proliferation of passing offenses QB coach is the most common path to being an OC and LB or DB coach the most common to being a DC. Check out the bios for current top-20 coordinators, you will be hard pressed to find coaches who moved up from coaching the line.

[DC 86]

I also think there are very few "career" OL and DL coaches in the college ranks. Most guys in that job expect to become coordinators in a "few years".

This is incorrect. OL and DL coaches are by far the most likely to coach those units for a long time without ascending to coordinator. Because of the proliferation of passing offenses QB coach is the most common path to being an OC and LB or DB coach the most common to being a DC. Check out the bios for current top-20 coordinators, you will be hard pressed to find coaches who moved up from coaching the line.