Odds of Hanging On
Pretty decent. The Foreign Service actually likes lifers and one thing the federal government isn't very good at is firing people. Remember the movie Office Space where that guy with the mustache had actually been fired years before, but because of an error in accounting he kept getting paid so he kept showing up? Do a few years of work in the government and you may encounter a few people like that.
Seriously, the odds of hanging on are pretty good. Another thing the government isn't very good at is downsizing, so it's pretty unlikely the State Department would ever have to deal with mass personnel layoffs. Since our foreign policy is sort of in flux at the moment, the need for diplomats isn't going away any time soon.