Coach K will be 75 by the time the season ends next spring. He makes anywhere from 7 to 10 million a year, depending on bonuses, and has for quite a while. He has won numerous conference and national championships at Duke, has had dozens of All American players come out of his program. After all of that, maybe the man is just burnt out on coaching kids that only want to use the program for a year, to advance their name and likeness and then go off to the NBA. Or maybe he just wants to spend time with his wife, kids and grandkids, and kick back, travel and not worry about whether some 18 year old HS kid is going to sign with you or some other team.
Retire and enjoy life, because before you never know how many more good healthy years you have left at 75.