I think that was the point. Evie and David couldn't take the immortality and everyone dying or moving on anymore, and that's why they always moved on. Sebastien clung to mortals, expecting them to always leave him behind. Jack didn't follow the rules of that world and stayed, and eventually paid the price. I'm kind of thinking of Jack the way I did what's-his-face (Elaine's husband) in Promethean Age: trying to be the heroic/romantic figure normal for the genre, but there's no real place for it. So he's the one who dies.

But then, I'm one of those who would usually rather the author stomp on my heart for a fitting ending than give me a happy or ambiguous one to avoid that.