What we had last season is a situation where star players had, to varying degrees, made their future plans known. Davis was clearly jumping to LA. Kyrie and Durant were planning on one of the NY teams. Kawhi was eager to go home to one of the teams in LA.
At that point, the soon to be cuckolded team is faced with a decision. Lose them for nothing or hold out on a slim hope they will stay. Typically, smart organizations don't risk it unless they think they can win a title. The Raptors and Warriors correctly estimated that their teams were good enough to win it all and were willing to take the risk. The Celtics did not have this burden. It's clear now that from the start, the team they had constructed simply wasn't good enough.
I would wager that half of that was misguided optimism surrounding their own roster and the other half a misread on just how good the other Eastern teams had gotten. Milwaukee made a huge leap by changing their coach and adding Lopez. The Raptors added Kawhi and Siakim jumped a tier. The Sixers made huge trades midseason to add two all-stars.
Obviously, timing is everything and hindsight is 20/20. Whether or not they should have had more foresight doesn't matter because the end result is ultimately what matters.
Where I really have questions about their judgement is, Kyrie was already by all accounts doing everything he could to ruin team chemistry. When you combine that with the reasonable assumption he already planned to leave, don't you have to consider it a lost cause?
The rationale we kept hearing was that Davis might convince Kyrie to stay, but it was crystal clear his goal to go to LA was not changing. If you already knew before the trade deadline Klutch and Davis had no intention of being more than a rental then you should have already made the decision whether to deal for him or not this offseason ahead of time.
Sure, a few things happened they couldn't predict: Lakers won the fourth pick. Demps, who along with Gayle Benson hated the Lakers for sabotaging their season, was fired and the more impartial David Griffin replaced him. However, Danny either changed his mind about how much he would offer or miscalculated what the price would be. According to reports, Ainge was only willing to offer Tatum and not much else. Meaning, he was willing to part with one, maybe two top assets but not multiple ones.
Maybe Ainge got gunshy because he learned this season it takes incredible maturity for a team to play well when they know their star is a rental. The Raptors never let Kawhi's pending free agency deter them on the court or in the locker room. The Warriors, besides one Draymond blowup, more or less ably navigated the season knowing Durant was likely gone. The Celtics' chemistry, on the other hand, went from bad to worse when it became clear Kyrie was gone. They already hated him when they thought he might stay, so why would they listen when it's clear he's gone?
Ainge loves taking risks and it's clear he was tempted to trade for Davis regardless of the risk and whether or not Kyrie stayed. But he underestimated what it would cost and probably realized he wasn't going to get the real Davis anyway. He would have been trading for the Davis who already ruined the Pelicans' 2018/19 season: the one with one foot out the door.