I've read most of these stories purporting to explain the team's issues, and they all leave me with the same feeling that the reporters are unwilling to do more than imply where the problems were really coming from. No specifics that we didn't see already.
I guess it's a matter of control. If a player isn't doing what the team wants, the team should be able to impose a correction onto that player. If that doesn't work, you have to know when the problem crosses a line that would cause team level deterioration. You get what you want from a player or you move the player. When you don't do that, the coach's authority is degraded.
Kyrie was the only player on the roster that Stevens had no real leverage with. Everyone else should have been manageable. The fact that Ainge didn't move anyone tells me it was his fault. Stevens reports to him, and ultimately, all of Stevens leverage with the players is derived from Ainge's power to demand performance or induce exit.
It's not that easy, professional sports simply doesn't work that way. The team's reputation with players is very important. If a GM or coach tried to run a team as you suggest, they would quickly find themselves losing credibility with the players. That has real consequences for team building. Even Popovich can't simply demand that sort of compliance from players, see Kawhi Leonard.
Please don't oversimplify what I said. Of course the players have a great deal of leverage. That is why judgement and team management matters so much. You can't act like you are a drill sergeant, but you do need to know when team cohesion becomes unstable enough to make the tough decisions you need to make. Otherwise, you have a billion dollar business held together by the whims of two or three guys who are untethered to any consequences.
Some players understand how much depends on their maturity and some of them don't. Management discretion becomes the difference between success and failure. This becomes more important in the modern NBA than ever before.
No one in the public knows what really happened in San Antonio. We don't know that Pop had a path to resolving Leonard's situation or not. We did see Leonard create a huge mess. I personally think Pop did the right thing, even knowing what we know now. Injuries affect both the player and the team, and it is necessary for both parties to cooperate. Leonard refused to do that.
Yes, sometimes immensely talented people become problems, and sometimes dealing with that takes away opportunities. Not dealing with problems takes away opportunities too. Management gets paid to decide which path is the wise one.
This wasn't some trivial level of acting out. The team as a whole was underperforming significantly. The ability to know when to correct and when to tolerate is the whole ballgame. If you believe that last year was a time to tolerate, fine. I think it reached a threshold where action was required.