It seems that some are judging based on what we knew at the time and some are looking at end result. Based on end result, it was a horrible deal. It was a two year detour and while it's easy to measure the cost of what we gave up in the trade, the actual cost to the franchise is harder to quantify. How do measure the negativity, the hit in our franchise's perception, the hit to Brad's reputation? Did adding Kyrie prevent us from other opportunities? Did it affect Horford and Rozier's decisions to depart? We'll never know how things would have played out.
There was also the negative publicity cost of trading Thomas while he was injured. That was a hit on Ainge and the franchise's reputation, fairly or unfairly. Things ended up looking even worse when Irving turned out to represent everything that was the opposite of Thomas in terms of attitude, leadership, and buying in to the city of Boston.
They didn't want to take a risk on certain stars who were older, or ones who were rumored to be rentals (George, Leonard, Butler, etc.). They ended up taking the risk on a player who was the biggest head case of them all and one who didn't even have the courtesy to keep quiet and pretend like he might stay, affecting the team's play on court.
Anyway my main point is that the trade not "costing" them much in assets is misleading. There were definite, immeasurable costs to what happened this past season.