I agree, a player that's 24 years old can still have some development in front of us, it's limited though. Players enter their prime at around 27 years old, before that there is time to shape their game and body. A player that is like 21 years old and puts him his numbers obviously would be FAR more interesting for a team in the NBA since he would have a much higher upside.
While Biberovic of course is a great player, he is on a different level skill and production wise cause European basketball is still way different compared to what's played in the NBA. If you want to make it there it's basically essential to make the move over to the states as early as possible. The longer you wait the smaller the window for success is becoming.