Fultz signed a 50 million dollar contract at age 19, and you are questioning if he has been successful basketball player? That's more money than you or I will ever see in our lives.
Depends on what he does with it. Iverson blew through much more than that. If you end up broke, doesn't matter how big that contract was.
If he doesn't get a big time second contract he didn't live up to his potential value. Who knows if he would have improved playing for an elite coach for a few more years.
The reason the one and done exists is because the NBA was getting far too many kids out of high school that were busts. That was costing the NBA money in two fronts. The kids weren't developing and it was dragging the overall talent down. Also, look at Biggie to Portland. Portland gained thousands of fans from Boiler nation buying jerseys and gear. The NBA absolutely benefits from kids going to college and honestly should look st making it 2-3 years out of high school. It will only benefit the NBA.