His Paris team is being elevated to Pro A this year,
Yes. Moving up a level is a reason to keep him over there. The top French league will see him in an environment that's more difficult than college, at the same age as most freshmen prospects. It should make his transition to the NBA easier, and also give the team a better feel for what level he should be at going forward.
The European players I guess follow a different path or progression. In the US, High School basketball is pretty structured for top players who often end up at private schools to play basketball, then there is the travel/AAU teams where they get more coaching and competition, then for the best, one or more years in college with top coaches and top competition.
For Juhann Begarin, I am guessing HS was not nearly as competitive so he goes to Pro B and then Pro A, then maybe G-League, then if all goes well, NBA. Berarin is 18, turns 19 in August so I think he would be just finishing high school if he was in the US and heading to college. He would be a fairly high recruit I would think. So now, instead of a year of college after HS, he gets a year of Pro A in Europe after a year of Pro B.
I would guess that the HS-College path would result in more development as he would be a top player. As a Pro B/Pro A, the teams are probably less concerned with his development but the competition is men, not boys. I think it is great that he is going to get a year of Euro Pro A at age 19. Let's see what he does with it.
I wonder why he declared this year and didn't wait a year. He is not all that far off from the first round and maybe after a year in Pro A, he would have been in the first round, getting first round money. At 19, he would still be plenty young for the draft. Now there is no way he can get first round money. Good for the Celtics though.