Tim Duncan is an old tired man, that's why he's only getting 28 min per game. I believe he's also getting more then the 2-3 points per game that Joel Anthony is getting, which kinda justifies him starting.
Millsap is much younger than Duncan, and if he 'were' good enough to start on a championship team he would demand more mins then that; which gives the other team plenty of time to take advantage of his shortness. (remember, your example Duncan is almost seven feet tall, strengthening my position.)
If this were true, than it wouldn't be Millsap that makes us a contender, it would be Dwight. All this is saying is that Millsap doesn't have what it takes to get it done without superstar help, so he'd be what, after a good point guard the third or fourth option. Yeah, every team needs good role players -like Millsap.
As for Charles Barkley, different player different time. Barkley did well at 6'7ish during a time that Karl Malone was dominating the league at 6'9. Now you have Pau Gasol at 7', Duncan at 6'11', Bosh at 6'11, Dirk at 6'11, all playing the majority of their time at the 4. In order to play the 4 for the majority of mins a player really needs to be at least 6'10, if not than that player is a LIABILITY.
Paul Millsap is a situational player. If the other team goes small, he's a great asset, and he may get significant mins playing that way, but if he were to start full time, enjoy the lottery, or first round exits.