NO WAY HE LEAVES FOR THE SUNS. PERIOD.
The average tenure for an NBA coach is ~2.5 years, and that is heavily influenced by Popovich. No matter what they offer, he'd have to go in knowing he's unemployed in 2 - 3 years and looking for a new home.
He's the toast of the town (Philly is a pro town, and all of their teams are all in rebuild mode). He's working in the area he was born and raised. Having won a NC at a school (technically, he's coached 2 NC teams) he's been at for 2 decades means he is guaranteed employment for life. He doesn't have to uproot his family or deal with the so-called "professional" athletes and their agents. He's set. He's making millions now and will make more regardless (salary, endorsements, media appearances, a book in the waiting, etc.). IMO, it's not about (more) money for a guy like this, it's about commitment, loyalty, and the knowledge that you are where you want to be, doing what you want to do. No one should throw that away for a few more shekels - he's too smart to get fooled that way.