Definitive Reason 1: Unparalleled Impact on Winning
The best way to have a high impact on winning is simply to win a lot to maximize one's opportunities to provide impact. And let's be clear, Duncan has a case as the winningest player in NBA history. His five championships, six finals appearances, 72 percent career win percentage, and 19 years of always making the playoffs represent the combination of winning at the ultimate level and sustained greatness.
Beyond these raw stats, Duncan is one of four players in NBA history to have at least three Finals MVPs and five championships -- Shaquille O'Neal, Magic Johnson, and Michael Jordan being the other three. That accomplishment ignores how Duncan ceded the spotlight to teammates in NBA Finals the Spurs were clearly going to win. And despite hypocritical assertions by his detractors, Bill Simmons highlights Duncan did this without an elite supporting cast and no costar within the top 15 players in the league when he captured at least his first four championships.
His role as the most important player on five titles runs ranks second to Jordan, who alone matched Duncan as a true two-way threat. And none of this quartet possessed the career longevity he had. I think we can agree Shaq's "Win a Ring For the King" and "Big Shamrock" runs in Cleveland and Boston were better forgotten.
Using this argument, Bill Russell also certainly has a case for winningest player ever, but the level of competition he faced versus what Duncan played against were two different beasts. While Russell competed against Jerry West, Elgin Baylor, Wilt Chamberlain, Oscar Robertson, and the tail end of Walt Frazier, Duncan had to battle late-career Jordan, Karl Malone, John Stockton, Gary Payton, Hakeem Olajuwon, and Patrick Ewing. Not to mention prime LeBron, Kobe Bryant, Shaquille O'Neal, Stephen Curry, Kevin Durant, Dirk Nowitzki, Kevin Garnett, and many others.