This question brings me back to that very first time I saw the film. Not only did I not know Jack was going to die, I didn't know any of the plot line. I remember, though, being drawn into that tactic used, of long, slow passages, that left a person almost sedated, followed by sudden unexpected punches. That pattern was, I think, necessary, to make the story mean more and more to the viewer and to advance them to the unexpected, relentlessly challenging, and almost sacred ending.
In that context, the flashback scenes of Jack's death were only one of those sudden punches. I also have to admit, I was so dazed, I couldn't follow the entire plot line until the 2nd time I saw it (which was as quickly as I could get someone to go with me). And, I know this sounds strange, but, I still haven't formally registered Jack as having died. Am actually in denial, over events.....in a film!