KG should not be the last line of defense that's for sure. He's still better than most PF in the NBA and can still contribute (as evident by 17+ rebounds three straight games).
We need someone who will go after the shots when someone penetrates. Someone who can take the blunt of the physicality against most Centers. And KG along with this someone, can patrol the defense and the rotations. KG can still be the pick and pop option.
Sully can back up KG at the 4 and move Bass at the SF (I'm sold that Bass can play the 3).