First, it's not the super max, it's the max. Huge difference. And if you will notice, every all star level player (and some non all stars) is getting the max these days. You see the comparisons to Luka's salary over the next three years. Well, when Luka comes up in 3 years, the Lakers are going to have pay him something like $83 million a year. I promise you Fox's next contract (if he's even still on the team) will look much differently. Secondly, Harper is 19 years old and has never played a minute of an NBA game. I doubt he's one of the best 25-30 players in the league right off the bat like Fox is, and there's almost zero chance he's the better player right now. The Spurs are trying to compete for the playoffs right now.
Also, they just locked up Fox for the absolute prime years of his career (age 28-32) and he has no option in year 4. If Harper turns out to be that much better than him and Fox becomes disposable, he's still a tradeable asset. And again, it doesn't blow their cap because the rest of their core is on rookie deals. Worst case is that they end up in the tax one season when Harper's extension kicks in but that's four years down the road.
With regard to Clutch... you also need to consider what the Spurs had to give up to get Fox, which was hardly anything for an all star point guard in his prime. Look what Orlando gave up for Bane. Look what the Knicks gave up for Bridges. You gotta give Clutch some credit for the way they played their hand in Sacramento. The Spurs gave back a highly protected Chicago FRP that was likely never to convey. They gave up an even more protected Charlotte FRP that ended up converting into two seconds. They gave up their own 2027 pick which should be in the 20's if Wemby can stay healthy. They gave up the Minnesota unprotected in 2031. They didn't even have to give Sac their own FRP pick back that they fleeced them out of to facilitate the Derozan/Barnes trade. And they got to unload Zach Collins and his shitty contract on top of that.
Would I have liked Fox to sign for less? Sure, but this is just the way business is done in the NBA today. I don't get the criticism at all. And like I said, I promise this extension was baked into the deal in the beginning when they made the trade. Now, I don't know that they make the trade in hindsight if they knew they were going to luck into the number 2 pick in the draft, but so what? Hindsight is 20/20. That they got Harper is gravy and yet another asset.