I don't think Ainge did much wrong at all, just ran bad which is the nature of being a GM in pro sports.
Kyrie trade was a no brainer at the time, it just didn't workout and he left but anyone is trading the 7th pick and Jae Crowder for him.
AD, Kawhi clearly wanted to live on the beach so no point in dealing a big haul for them (which is why Toronto didn't have to give up much)
Kemba was the best guy they could sign so they did, that's usually what you should do and figure the rest out as you go rather than split the $ up with 2-3 smaller contract players. He's overpaid now but that offseason there wasn't anyone else to get. You can't save the $ as Tatum/Brown were due extensions to put them over the cap anyway.
Over time if you can't cash the chips in for the best players in the league then you regress.
Ainge/Stevens stopped finding/developing the diamonds in the rough the last few years that they had been able to find late in the draft or via FA/trade in previous years (I Thomas, Bradley, Rozier, Morris, Crowder) but that is also pretty hard to sustain. You see teams do that in a short time span and it seems like they have found the magic potion but a lot of times it is just variance (think the Legion of Boom most notably, they drafted like 5 all world D guys in 3 years and none since) or other teams improve their scouting/tactics.
Lastly, Stevens is a pretty good coach but he also felt the regression of coaching. He was way early to a lot of the changes in the game that he was coaching circles around the other coaches (it helped the East was really bad until around 2018-2019) but eventually they caught up to him too. The edges the best coaches have now is a lot smaller than it was 5-7 years ago when he started his run.
