Arguing against Duncan as 'the best player since Jordan' is pretty easy really, if you go off individual performance / stats - which I think is a pretty normal think to do when you're asking who's been the best 'individual' player in that time. Duncan's stats have been good, but there are other guys have had better stats then he has.
All the arguments about Duncan's greatness ultimately come down to the number of rings he has, and that's team (not individual) success.
An interesting point...
In the last 30 years, there have only been three franchises who have managed to win 3+ championships within a 5 year span:
LA Lakers (85, 87, 88) and (00, 01, 02)
Chicago Bulls (91, 92, 93) and (96, 97, 98)
San Antonio Spurs (03, 05, 07) or (05, 07, 09)
Question:
What is something all three of those franchises (across each of those seasons) had in common?
Answer:
They all had perennial hall of fame coaches - Pat Riley, Phil Jackson, Greg Popovich.
People can argue that Tim Duncan was the only consistently great thing on those Spurs teams (and hence he gets the credit for that success) but that's not entirely true. Every single one of those titles had Popovich at coach, and you can't underestimate the importance of that. You could almost argue Popovich is the Spurs second star.
For example, look at how well Boston did this season - how much of that success is because of Steven's ability to get everybody on the same page?
Now much as I dislike Lebron (and I really do dislike him, as a person) the Heat made it to the finals 4 years in a row while he was there, and won in two of those four years. That's probably the most success we've seen in the past 30 from a team who didn't have a world class coach.
Even the other back to back winners during that time (89, 90 Pistons and 94, 95 Rockets) had top tier coaches in Chuck Daily and Rudy Tomjanovic.
Lebron is now with a new team and he's about to go back to the finals again...and much like in Miami, he doesn't have an especially highly celebrated coach.
Don't like the guy, but credit where it's due.