I don't really buy this idea that Brad Stevens can't handle a 'big personality'. He worked just fine with IT, who is a huge personality and has generally been a malcontent at most of his other NBA stops.
I'm mostly inclined to write this season off as a perfect storm. We had a lot of subplots, and most of them didn't play out well. Our young group, coming off of an overachieving 2018, thought they deserved larger roles than they got and then rankled when 'their' minutes and assignments went to Hayward. Brown eventually found his place, but Tatum and especially Rozier never did. Hayward, for his part, looked like a shell of himself until about 80% of the way through the season. He showed flashes of the real Hayward, but no consistency. Kyrie spent a large part of the season practicing his sulk face, and the AD bombshell shook our locker room up hard.
And yes, Stevens failed. I agree with that. I don't think any coach would have succeeded in this circus this year. It is what it is. He'll have a shot to make whatever adjustments he needs to make and try this again in a few months. Kyrie, Tatum, Hayward, same thing. This chapter isn't our last.