This OP makes no sense to me.
Tatum was the most skilled and NBA ready player in the draft, at 19 years old. That's why Danny drafted him.
It has nothing to do with Giannis, who was essentially a raw physical specimen who looked like a 2-3 year project at draft time.
Tatum from day one had NBA ready offensive skills, and a natural ability to create offense, and he had the physical profile to be able to play the PF spot later down the track. They were all major needs for Boston.
When you have a guy available who has that type of scoring talent combined with solid defensive upside and plays a position of need all at the age of 19 - you take it.
The Jalen Brown pick had some Giannis parallels, but the tatum pick with classic Danny. He was the safest pick while at the same time having as much upside as anybody else in the draft, and had the typical personality (hard working, highly coachable, team first guy) for a Danny Ainge pick. He pretty much had Danny written all over him.
A perfect example of why Danny chose Olynyk over Giannis would be to look at Jajuan Johnson versus Etwaun Moore. Both taken in the and draft, but JJJ was taken higher, as the high risk / high reward prospect... Versus Etwaun who was a solid player but perceived as having little upside.
Now JJJ has been out of the league for half a decade while Etwaun made modest contributions from day one, has been a solid rotation player for years, and is currently starting for an NBA team.
Giannis became Giannis, but he just as easily could have become JJJ. Olynyk was Etwaun - he's been contributing to NBA teams since the day he got in to the league, and likely will for another 8-10 years.
Tatum is in the middle. He has Kelly Olynyk readiness with Paul Pierce upside.