That or a salary increase. Only those two reasons why a player would decline the player option. He is valuable now. There are lesser point guards in the league who are healthy and he is one of the best choices of any team who doesn't have a playmaker because he can score on his own, handle the ball with care, and he can make a play too. It's hard to find guys who could do both.
Players who reject a huge offer likely believe that they can still make a lot of money compared to the offer on the table. VanVleet was the leader of the Raptors last season, and his stats were very consistent. He can certainly contribute that to other teams as well. He desires a max offer, and with the Raptors having a new coach, there's no assurance that the current system will continue to work in his favor.
Or the fact that they fired their Head Coach might be another reason as well. Another is maybe he don't want to be in the Raptors anymore thus, he is trying to be a free agent, and with his elite ball handling skills alongside good three point shooting, I'm pretty sure that there will always be a team that will get him. The only question right now is, which team will acquire him, and will he get higher salary than what he can get in the Raptors which is $28M.
Max offer/contract for him? I don't think so though he might be eligible for around $100M - 4 year contract, or even higher if somebody will try, and give it to him.
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To summarize, Bronny James' eligibility in the NBA Draft will take place in the next year's draft (2024) for the season 2024-25.
I guess this is also the reason why LBJ has a Player Option for the '24-'25 season, so that he can just opt out, on his contract, and follow his son because it has been his dream for a long time. On the other hand, he said in one interview that he is still ok if that thing will not happen. That him, and Bronny will not play in the same team.