Maybe more so; there really are two main defenses (3-4 and cover2) so defenses coordinators are maybe more expendable And at the same time there is basically a never ending supply of OC, QB coach, passing game coordinator, running backs coaches, wide receivers coaches and on and on that have all come from similar backgrounds, and if your guys go you either next man up or poach from another staff with similar backgrounds. If you have a good head coach with a defensive mind who brings in an OC that sets up the entire offensive system and gets a quality QB, you should be good. And if that QB is actually quality, and if your head coach is actually good, you should not have a problem resupplying if/when that OC moves on to a HC job. Usually you get more than one year and can judge who on your own staff is worth the promotion. Even if the OC is one and done a good HC should have a number of names in mind to make a more or less seamless transition. And it doesn't have to even be an entirely new system.