I don’t see it being Kirksey in isolation. I think RAC’s D has regressed season on season for 3 or 4 years now. I don’t think JJ Watt has played at the level you think he has during that time, but I do think he was still playing the same way he did when he was going for the sack record.
I’m not really sold on Lovie Smith as DC either as it happens, but I think this 1 year roster build can set us up for the future to play disciplined, to not let every RB with a pulse run 200 yards on us, and maybe slow down TE’s also.
Easy to get mixed up between what JJ was his first 5 years and what he was more recently. The on field result from the D as a whole, there isn’t much room to get worse.