The hard part with this is that the "expected value" if you will of the top end of the draft is much higher than the mid to late portion of the draft. The Athletic had a nice statistical analysis of this in their draft mega-issue.
I don't remember the exact numbers, but the general idea is that most of the players in the top part of the draft should become useful NBA players and a minority, but decent proportion, even become stars. So, while Ainge gets credit for drafting guys in the top 3 who did turn out to be stars, it isn't that hard.
On the flip side, a guy drafted 15-20 has a pretty low likelihood of being a star - or even a useful player. So, if Ainge has struck out on those, the expectation wasn't that high to begin with. Just to make up numbers, if 0 of 10 players drafted by Ainge in that range were good, the expectation might have been that only 1 or 2 would have been good on average. Going from 1 to 0 is basically statistical noise.