With the money those voice actors are making they could have retired ages ago, but why?
With that kinda money for work that comes almost second nature, I would do it till I couldn't do it anymore.
But if they are smart, they will negotiate for one more year at a large paycut and a larger slice of merchandising, syndication DVD sales on the back end.
And throw in a few direct to DVD movies/specials/Treehouse of Horrors episodes down the line.
They gotta milk this thing till the teet is bone dry and keep that residual income flowing in.