I'm sure to some extent Ainge and Stevens fell into the same trap that a lot of fans fell into.
Flashback to summer of 2017, and we have Gordon Hayward, an All Star, joining the Celtics in place of Olynyk. He picked up Baynes as a free agent, who was a perfect fit next to Al for that team. Then Kyrie falls into his lap, which also allows him to move from IT4. Ainge had assembled a "mini-Big 3" in Al/Kyrie/Hayward, and had a promising 2nd year player in Brown and an enticing draft pick in Tatum. And he turned Avery Bradley into a Marcus Morris who was recovering from a major knee injury but who would be able to play a role coming off the bench his first year. And Rozier was the backup PG for the Kyrie maintenance days. Everything seemed fine during training camp that year. And then after Hayward's injury, the team goes on a 16 game winning streak en route to a really good season with Kyrie at the helm through early March. Then the playoff run happens, with Rozier, Brown, and Tatum all playing key roles, and the future looks really good.
Is that a team anyone would expect Ainge to blow up by trading Kyrie? Was it time to trade their young players for a player coming off a serious injury who was clearly committed to playing in LA once he became a free agent? Honestly, let's be realistic; everything on that Celtics team was pointing up and to the right.
Ainge kept Rozier, mainly as Kyrie insurance, as Kyrie's knee injury was worrisome at the time. And, to a lesser extent, Morris was kept as Hayward insurance. But is it really a slam dunk that the team would have been better had either or both been traded? Rozier was a backup PG; how much difference would a different backup PG have made? Morris was a key player for the team during the first half of the season, especially with both Brown and Hayward struggling. I guess Ainge could have traded them at the trade deadline, but the Celtics still don't make it past the Bucks.
Kyrie was not the only problem with the team. But, unfortunately, he was one of the problems, and it's definitely best for all parties involved that he moved on. The roster redundancies have been addressed with Rozier and Morris moving on. I think it's prudent to give everyone a clean slate and see how Brad and Danny do going forward. So far, Ainge's decisions since the draft and the start of free agency seem to have been the right ones, but time will tell.