Duncan didn't change the game the way Shaq did. Neither did Robinson. And, if Olajuwon is your standard, then your Hall Of Fame is going to be limited to players who were as good, or better, than Olajuwon. Robinson, Barkley and Drexler fall short of that criteria.
OK, I'm going to play devils advocate for a minute, and then I'll let it go. On one hand, you could say Shaq changed the game simply because of his being a freak athlete. There aren't many players that come along that are 7'1" without shoes, that have a 36" vertical, and can run like the wind. On the other side, one could argue that Shaq did little to improve his game, simply because he didn't have to. He today, and always has been a very bad freethrow shooter. He has little or no game away from the basket.
O'Neal averaged 24.7 PPG, 11.2 RPG, 2.6 APG, and 2.3 BPG through last year. He won ROY, and was voted NBA MVP one time. He's been voted into the all star game 15 times out of 19 seasons, and has 4 championship rings. He was voted on to the NBA all defensive team 3 times. Throughout his career he's averaged 65 games a season.
Duncan averaged 21.4 PPG, 11.7 RPG, 3.2 APG, and 2.3 BPG. He won ROY, and was voted NBA MVP twice. He's been voted into the all star game 13 times out of 14 seasons. He has 4 championship rings. He's been voted on to the NBA all defensive team 13 times. He is the only player in NBA history to be voted onto the all NBA team, and the NBA all defensive team 13 years in a row.
Now aside from getting caught up in the size and strength of O'Neal, and the fact that he's a one of a kind specimen, I think Duncan's results match up quite well if in some areas aren't better than Shaq's. Duncan also averages playing in 74 games a year. So he's also proven to be more durable than Shaq.
In my humble opinion, if Shaq belongs, then so does Duncan.