First, we are selfish, and don't want to die (no matter how useful we are to society). Second, we also care about a few other people close to us, and don't want them to die. Third, we want to spare everyone from having to be afraid of death.

I think if you forget about these reasons, then there's no point in preserving people.

Edit: I'm sorry that your comment was downvoted, but I for one think that it's a worthwhile objection to make, even though I disagree with it for the above reasons.

What is it that is lost when a person dies, that cannot be regained by creating a new one?

I'm uncertain about the value and fungibility of human life. Emotions clearly support non-fungibility, in particular concerning your own life, and it's a strong argument. On the other hand, my goals are sufficiently similar to everyone else's goals that loss of my life wouldn't prevent my goals from controlling the world, it will be done through others. Only existential disaster or severe value drift would prevent my goals from controlling the world.

(The negative response to your comment may be explained by the fact that you appear to be expressing confidence in the unusual solution (that value of life is low) to this difficult question without giving an argument for that position. At best the points you've made are arguments in support of uncertainty in the position that the value of life is very high, not strong enough to support the claim that it's low. If your claim is that we shouldn't be that certain, you should clarify by stating that more explicitly. If your claim is that the value of life is low, the argument your are making should be stronger, or else there is no point in insisting on that claim, even if that happens to be your position, since absent argument it won't be successfully instilled in others.)

What is it that is lost when a person dies, that cannot be regained by creating a new one?

If you go to a really high place, and look over the edge far enough, you'll find out.