The D was very good statistically in 2018, Jimmy’s first year calling plays. Personally I think Coach K is a better play caller and overall DC, but Jimmy probably would have improved over time. Last year’s struggles had more to do with the gaps in the roster than scheme or talent.
Jimmy has a personality that is way more suited for today’s game, I’ve also never gotten the impression that Coach K had the goal of being a HC. He would have left a long time ago.