To be clear, I thought the Rozier contract (and the Hayward contract) were dumb. I still think they are, in the abstract. But Boston thinking that paying Walker max money was a good idea, even if Walker stayed healthy, was just dumb. Boston's goal is to win a championship. Walker can't be one of the 3 best players on a championship team, even if he is healthy, because of his defensive limitations. And that was before you consider that the Hornets didn't make a max offer to Walker, and that they would be in the best position to know his medical risks.
Ainge is going to end up with a really complicated legacy. Put together the 08 title team (with some help from McHale), fleeced the Nets, was smart enough to pick Brown and Tatum, and got royally screwed with how Hayward's time in Boston went. He also hired Stevens and obtained Irving. But the rest of the moves, yikes.