I don’t see why there wouldn’t be.
Apart from, maybe, catastrophic world events, perhaps involving the death of most humans by war, disease, or artificial intelligence, I can’t envisage what would stop them keeping on appointing future GB members.
There’s a saying attributed to the Marxist theorist Frederic Jameson that it’s easier to imagine the end of the world than it is to imagine the end of capitalism. Maybe I’m caught by the Watchtower equivalent: it’s easier to imagine the end of the world than it is to imagine the end of the Watchtower organisation.