I wonder if , at some point, Fitz will start thinking legacy and identify the person he wants to take over for him and start laying the groundwork for that, both internally (within the program itself) and externally (within the athletic department and University). The higher the likelihood that Fitz is succeeded by someone internally, the larger number of people associated with the program keep jobs that would be immediately at risk if you bring in a complete outsider (one of the reasons coaches like Bryant and Schembechler stayed so long, and in the case of the latter, got named AD to avoid all uncertainties.
All of the above happens only as long as the winning continues.
One might argue this is what a Randy Walker was starting to do with Fitz (especially internally) before his death.
It is a challenge to burn very bright and very hot for such a long time. It is helped if one has created a life for one’s self outside of football.
As an aside, the success of Northwestern athletics in the long term is largely dependent upon on the TV contract. If in the next round of renegotiations (mid 2020s) the contract number shows signs of weakness (only holding steady or declining in light of cord cutting tendencies), that might be a big motivation to tie a coaching exit strategy to peak roster/performance and retire sooner than later.