Given that this is a trend shared by just about all contending teams, I think there's more to it than meets the eye. Even the Spurs just kept recycling retreads again and again during the primes of Duncan/Manu/Parker. I think familiarity and experience may matter more than actual talent here.
Young guys are so hit and miss. How many Jordan Farmers, Luke Waltons, Vuvejecs, etc. do you have to cycle through to get a Trevor Ariza? I think if the NBA had a farm system that actually features talent(like AAA in baseball) things would probably work better.