In a nicely done article on the rise of point guards in the NBA (with some praise of Rajon Rondo of course), I was struck by this section on the decline of some star power forwards.
As we welcome this exceptional class of point guards, we say goodbye to the most talented group of power forwards the game has seen. Chris Webber is at the beginning of what proves to be a promising broadcast career, Tim Duncan and Kevin Garnett are getting older, slower, and less effective with each passing season, and Rasheed Wallace has resigned himself to harassing referees in semi professional summer leagues. As Charles Barkley often reminds us, only one person is and will forever remain undefeated, "Father Time". The league will miss Duncan’s understated dominance, Garnett’s relentless drive, Webber’s uncanny vision, and Sheed’s exuberant personality.
You can argue all you want that these guys aren't dead yet (cue Monty Python skit), but the point remains that they are closer to done than not. And in fact, I'd maybe toss Dirk Nowitzki in this era - even though he's got a few years left and obviously hasn't lost much off his fastball.
Also see this thoughtful piece of future-fiction about Ray Allen's final game next season.