I think with K in the later years you have to take the whole package, you get the larger than life legend who could practically pick and choose his recruits, but you also get the old man past his prime who can no longer has the energy or the drive to squeeze every ounce out of his roster.
You can't separate his teams' talent level from the fact that his legendary status is the culmination of 30+ years of accomplishment that leaves him the old and tired man that he was. And I don't think the issue was ever defensive schemes. Heck he had Scheyer on his staff, surely if the issue was just the knowledge of schemes they could have run the same ones last year as the ones they are running this year.
The fact of the matter is you're dealing with 18-19 year olds, and to get them to have the discipline to play good defense consistently for 30 secs each possession takes more than just schemes, it takes relentless will to beat the good defensive habits into them.