I strongly doubt that the creators of Lost will ever kill off Jack, unless they kill the entire cast.
For one reason, he is sort of the main character. Not my favorite, but there's no getting around it - he's the big guy. He was the first character in the first episode, so it was kind of hard to get rid of him after that.
Also, if you look at the case for the first season of Lost, you'll see the group sillhouettes, then on the side, you'll see Jack.
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