That is my confusion. I don't even have a dog in this fight. I am just trying to figure out what the argument you are trying to make actually is.
Are we talking purely about stats? If we are, then I would tend to agree with you as a generalization most players have declining statistics after when they are in their 30s although there are probably some player who this is not true for. If we are talking about importance to their team / overall play then I think there are several examples that disprove this. Proof and others have already given several.
Stats of course, but not only. That is beyond evident.
Most players decline physically as well. This is also very evident, but not as simple to see as the stats. Look at minutes per game. They often decline the same time. Read interviews and books about NBA players and you will often find in their own words how they just don't "bounce back" between games like they did in their 20's. The fact that around 90% of NBA players do not play past their 33rd year speaks volumns about this.
You point to Duncan as an example. He's not. He's one of the exceptions to the rule. He's one of the best players of all time, same as LeBron. You guys want to put Marc in that same conversation? C'mon.
Duncan has also had the luxury of being on the most stable, best run franchise for the last 2 decades, not to mention the talent surrounding him. All that makes it easier for him to age more gracefully basketball wise.
Of course there are exceptions to every rule, but most of the rare times that players pick it up for a season or two when into their 30's is because of other factors such as better teammates added making things easier, new coaches and/or systems, among some. An excellent example is the Paul Westhead Nuggets 90/91. I always remember how he made Orlando Woolridge look like an all star with his 25 ppg. There was another player as well that scored about the same for that year because of Westheads coaching style. Short lived though.
The average NBA career is only around 4 1/2 years, which also very obviously says that the average NBA player doesn't even make it into his 30's in the league.
Even if you find half a dozen guys that pick it up for a season after they are 30, how many seasons and total players encompassed that half dozen? Quite a few.
At best you will find a few exceptions, and very few when put into context of the nearly 500 men that suit up in a NBA uniform every season.