I don't think there is a Duke fan on the planet who thinks Duke won't see some sort of drop off after K leaves. He is arguably the best coach in the history of the game, so yes, chances are we will see a drop off
That said, a lot of Cats fans here are saying Duke is going to fade away into nothing, without actually having anything to back that up. Obviously no Duke fan will agree with that, especially given that Duke was a national power before K arrived, has been crafted into quite the brand over the last 30 years, and would be seen as maybe the most desirable coaching job on the planet when K goes.