?
In my opinion it is a number of factors. A supermax deal restricts your ability to sign other players and manage the salary cap and if it is for a player that has missed the last two playoffs then you think long and hard about dedicating that much salary to one player. I have no idea if the spurs can "afford" the supermax and to go over the luxury cap. I would assume they can but that it really cuts into their profits.
I'm not saying Kawhi is not deserving. I think all of the players are deserving of as much as they can get from the system. Players should not be asked to give up any more than owners and vice versa, because players like Kawhi MAKE the NBA. That being said, in order for teams to be compe ive, there has to be a good level of fiscal responsibility regarding the cap, good drafting, luck, and some sacrifice from the players to make room under the cap (see Duncan, David, Manu). Do they have to? No, but they do if they want to win Championships.
Also, don't forget after the last economy crash, the cap level actually went down. There WILL be another crisis like 2008 and when it happens, there could be some serious regret from teams with these kinds of contracts