For me - this question always boils down to opportunity cost. Who we drafted versus who we should have (with the understanding that hindsight is 20/20).
Unfortunately, drafting Olynyk over Giannis was legacy altering. Yes, KO went on to be a serviceable rotation big. But Giannis is a generational talent - an MVP and DPOY who can serve as the anchor to a team with title aspirations.
The Celtics scouted the Greek Freak pretty extensively. In the end, they went with a player with a higher floor, but much lower ceiling. My quibble with this move - the Celtics had just traded away KG and Pierce. The rebuild was well underway. Given that context, that just seemed like the right time to take a flyer on a player like Giannis, with a middle of the first round pick. Olynyk's selection was the conservative approach. For a team that struggles to recruit the truly elite players - you have to roll the dice in the draft.
Compunding matters, they traded two 2nd rounders to move up to 13. One of those was a 2014 Brooklyn 2nd round pick which wound up 34th overall in 2014. Nikola Jokic was selected 7 picks later at 41st. Either of these two players would have turned into cornerstones, with Brown and Tatum coming later via the Nets lottery picks.
If the Celtics had been able to draft either of these guys - we're viewing Ainge very differently right now.