Not all of the talent leaves voluntarily, though. They got rid of a lot of talent by firing them (they call it "releasing" them). Even people who have been with the company for many years are "released". Not that long ago they got rid of Kozlov and he has been with the company since forever. I guess its probably just as well because they never did much with his character anyway. They could have, but they didn't.
I realize there is only so much air time, but it really doesn't help when there is a small select group of wrestlers like Cena who hog all the limelight and prevent the new comers from ever getting a chance to be established.
Oh, and another good example is Mark Henry. He has been with the company since the Clinton administration, but only recently did they give him his due. I don't know if he's really the world's strongest man, but he really is a beast and he deserved to kick some ass. It isn't right that it took 15 years for that to happen, though.
And then what about Christian? Here's another great wrestler who was never champion until recently. Now he's a "two time" champion, but those two times happened just a short time apart and they were both very, very brief. Meanwhile Randy Orton gets his thing for the umpteenth time.
Meanwhile, there are wrestlers like Alberto Del Rio who show up out of nowhere and then a few months later they are WWE champion.