There are a myriad of problems Millsap has had to figure out that are unique to his more prominent role and minutes on the team
-He is undersized. He's strong enough, he's long enough, but only just and he's very short for the position.
-To compensate, he has had to outwork opponents as his bread-and-butter and do it against inferior competition
-Since he now plays against starters who are much more skilled and much more physically able (taller, faster, stronger, longer). Outworking those players night in and night out is much more difficult.
-Furthermore, he is firmly in the opponents scouting report/gameplan (or he should be, I can't understand why they give him his automatic top-of-the-key jumper seemingly every time).
-Outworking opponents every night becomes unrealistic when adding in the factor that he's playing 10 more MPG, again, against much better competition.
-He's had to compensate again by pacing his game, becoming more cerebral, and adding finesse and range to his game. He's done a very, very impressive job with that.
-Lost in the shuffle of all of this has been his rebounding. 8 RPG doesn't sound bad until you factor in the minutes he's played. He's actually below-average in % of rebounds grabbed and rebound rate for a big in the NBA. When you look at the competition, the scouting, and his pacing his game, maybe it was inevitable. But that doesn't make it any less frustrating. However, if Kevin Love can look like Dennis Rodman, I see no reason Millsap can't rebound more (their measurements are eerily similar).
Millsap has come a long way. He's far better than I think anyone realistically thought he could be. But he's not a pillar. If he didn't embody the spirit of this team and it's history (or at least what it should strive to be), I'd consider trading him for a true pillar. But for me, he's close to untouchable, even if he's not that other pillar.