I think its just a case here that the team/front office really likes Brad stevens and his basketball mind but at the same time he lost the locker-room. So they want to keep his talent around and maybe the front office is a better role for him behind the scenes building a team around tatum/brown.
Also, while Ainge has been a great GM fairly or not, I think his reputation has ruined him in recent years. Players don't seem to trust him because of the Isaiah Thomas situation and other teams are weary of dealing with him due to his reputation of wanting to "win" every trade. We will see if Brad stevens can alay those fears.
for whatever reason, the team didn't respond to his criticism. When he had young inexperienced guys they listened to him. Now that they are becoming stars and veterans they are tuning him out. So, it makes sense to me that we would want to shift to a "rah rah" ex-player type coach who might not be the best with X's and O's as brad was but can get in these guys ears.
its one thing for a tatum brown or smart to not listen to brad stevens who never played the game professionally. but if you have someone who played the game and was successful at it telling you that you are wrong? thats different