You keep missing my point, so I must be failing to communicate well. Let me start over.
When either unit of your team is successful, those coordinators are ripe for the picking for head coach gigs elsewhere, so you'll experience turnover and have to keep hiring good people. That is hard to sustain. So you ideally want to minimize turnover in the people running your units.
OK, now, most head coaches run a unit and are responsible for the scheme. So when those coordinators get hired away, you're not losing the person who runs the unit and is responsible for the scheme. So you maintain continuity on that side of the ball.
Most schools want to hire offensive minded head coaches, so you're going to be likely to experience more turnover of the people running your schemes on offense if you have a defensive minded head coach than you will on defense if you have an offensive minded head coach. Simply because offensive guys get hired as head coaches more often, so those candidates get cycled through faster.
You were referencing Lewis as if WE had him as a head coach, we did not. When he left, we maintained our theoretical continuity on offense under our offensive head coach in Babers. The issue is Dino hasn't been successful overseeing the offense here and has churned through coordinators, becoming more of a "CEO" model head coach. I don't think CEO model head coaches are very often sustainable outside the powerhouse schools that can pay top dollar to retain top coordinators and replace them with top talent when they leave. And Dino isn't even a good CEO model head coach.
Anyway the point is, there's a tradeoff to hiring a defensive minded head coach - you're probably going to have a hard time keeping a successful offensive coordinator for more than two years of success before they get hired away, which means you're going to have to overhaul the scheme/system/terminology every couple of years or limit yourself to promoting from within which is also risky.
