He's not an elite NBA superstar. He's a product of the Popovich system. Same way Tom Brady is a product of the Belicheck system. In 2017, the Spurs (without Duncan) won 61 games and advanced to the Western Conference Finals where they were leading the Warriors by 20 in game 1 before an injury to Kawhi Leonard cost them the series. If Kawhi doesn't get hurt, who knows, maybe the Spurs beat the Warriors and win the championship. In 2008, Brady was injured for the season and the Patriots still won 11 games with some guy named Matt Cassel.
The Spurs were great with or without Duncan. The Patriots were great with or without Brady.
The success of the Spurs and Patriots for the last 20 years is a result of of the coach's greatness rather than the players (Duncan or Brady).