Probst says that he's leaving so that he can spend more time with his future ex-wife Julie Berry (hey, I kid...), who was a previous contestant on the show, and that he's getting weary of living for months at a time in the exotic locations.
I don't doubt him, but I think he may also be getting out before the entire show goes belly-up. This is the 11th season for Survivor, and personally I've finally lost interest in the show and will not be watching it for the rest of the season. The concept is just getting tired and no matter how many twists they throw in now, it just seems to be more of the same old same old.
If he does in fact leave that is no doubt the swan song for the series... if someone new does come in, I'm sure the series will last no more than one or two more seasons max.
It was fun for a while, but now we have Lost, which is much more interesting. :-)