Again the number of those players is very, very limited. It's not only players of that level who get paid huge money.
Can they get someone better, not likely. So if they want to compete, they need to pay the best players they have. The money needs to go somewhere, better him than many other options. Look at the type of money FA's pulled who were not even big names.
Is Conley your "franchise"? he's good but he's not that level, but look what he got paid. That's the market. There's only a few LeBron, KD, Westbrook, Curry level guys around. IT isn't that level, but it isn't only that level that gets paid.
Look at what DeRozan gets paid, he isn't in that true franchise player status imo. Look at what a Beal, Parsons, aging Wade, Barnes, Batum get paid. Those guys are all 20 million plus. That's the landscape now.
Further to that, good luck selling your fan base on ditching IT if you're the Celtics and you can't bring in someone better than him as a replacement.