Whott, you're making a point that may be valid but is not really relevant to the point of this thread. Robinson was probably the more gifted player compared to Duncan. He was a rare physical specimen to start with, and has better regular season stats. However, the "greatness" of a player in the context of most player comparisons and indeed this top 30 list is not about who is more talented, but whose body of work is more impressive when all is said and done.
At this point in time, Duncan's body of work outshines Robinson's, and he isn't finished yet. He is universally recognized as a top 3 PF of all time, a top 15 player of all time, a 3-time Finals MVP, and ups his game in the playoffs every year. That counts more than Robinson's achievements.
To put this argument in perspective, compare the body of work of Dominique Wilkins and Scottie Pippen. The latter is included in the 50 greatest players of all time, and his body of work is recognized as far superior ro Nique's. Nique was clearly a way better offensive player, and his stats and talent far outstrip Pip's. Pip was a great perimeter defender, but defense is a team concept, and Pippen was fortunate to play in one of the best defensive teams of all time. Moreover, Scottie was a sidekick, whereas Nique shouldered a bigger load. Maybe Nique's teams were not good enough to give him the opportunity to showcase his ability in playoff situations. But people don't hold the fact against Pippen that he played with the GOAT. Fact is, Pippen got to play 6 NBA Finals and got the opportunity to show his stuff on the biggest stage. We'll never know what Nique would have done if he had been MJ's sidekick, but that doesn't count. You can't play "what if" games when comparing a player's body of work to another. Pippen did what he did, Nique never got the chance. Pippen > Nique.
You can't guess what Robinson would have done with Duncan's teams or vice-versa. Duncan got his chance, and showed his stuff. Robinson didn't, and that's his bad luck. But that doesn't give him any sympathy points.