Before its all said and done his estimated earnings from professional basketball will have been $7,000,000+ (conservative). Simple math shows that it would take someone 30 years at $233,333 per year to earn that.
So its not about how much he will make over the next 20 years. Its more about how financially smart he is. The amounts stated are enough monies for one to live for a long time depending on investment strategy and budget.
The argument for staying is rarely a sound economic decision. The opportunity cost is a college education which usually can't garner the financial benefit of being a 1st round NBA draft pick. For a lottery pick, the argument loses all of its weight imo.
Interestingly, its the pundits, coaches and fans that argue against guys going and rarely, if ever, the guys who got paid.