Spags has an all time great DC resume. He's in an amazing spot.
Handful of SB rings he was a huge part of winning, known as one of the best at what he does, can focus on D and not have the pressure and scrutiny of the head man, tons of money in the bank, beloved by fans, and 100% security in his gig.
Alternative is HC of a losing team with no QB and having someone else run the D.