The writers have always sort of hinted that there were other survivors (the one lady who "knew" that her husband was still alive). I think what they have done is give themselves a fresh new set of story lines to explore for this series. Also, sets up a huge conflict between the two strong willed leaders, the male Jack and the female Anna-Lucia.
But the writers had better wise up and quit killing off characters, at least characters that people might be interested in. Killing off Boone was okay, but killing off Shannon really bummed me out - she was to have kept the dog for Walt and she was Sayid's love interest and the connection to the aforementioned Boone.
We'll see how it goes, but the writers are in quicksand as far as I'm concerned.