By the age of 24, Simmons had already won Rookie of the Year, 3-time All-Star, 2-time All-Defensive, 1-time All-NBA, and finished 2nd in DPOY voting.
As a rookie he averaged 16-8-8 on 55% shooting with nearly 2 steals a game and won Rookie of the Year.
In 2019 he averaged 17-9-8 on 56% shooting and made the All-Star team.
In 2020 he averaged 16-8-8 on 58% shooting, led the league in steals, made 1st-team All-Defensive, 3rd-team All-NBA, All-Star, and finished 4th in DPOY voting.
In 2021 he averaged 14-7-7 on 56% shooting, All-Star, 1st-team All-Defensive, 2nd in DPOY voting, and got an MVP vote.
He also advanced to the ECSF in 3 out of 4 seasons, with career playoff averages of 14-8-8 on 57% shooting.
Before the injuries and mental issues, he was on a clear path for a Hall of Fame career. You were talking about someone on pace for 10 or more All-Star games, numerous All-Defensive selections, a handful of All-NBA teams, and might have snuck a DPOY at some point. He had ELITE talent as an NBA player before his mind and body collapsed on him.