(A) would strengthen the argument, but it’s not the principle underlying the argument. Even if (A) was false, the argument could still be true. For instance, the argument would still work if we only have a moral obligation to help future generations, not all other people.

We don’t know whether past generations preserved books for us. It could be that past generations often destroyed books, but that we have an obligation to preserve them.

The argument isn’t comparing our moral commitments to future generations with our moral commitments to the present generation. It’s just saying we have a moral obligation to the future — not saying that obligation is stronger than our obligation to the present.

This is the key principle. Without (D), it could be that we don’t have any moral obligation to future generations. (D) tells us that we do, and bridges the premise that books will enrich future generations with the conclusion that we have a moral obligation to preserve them.

The argument concludes that we are morally obligated to not destroy books. (E) just gives a necessary condition for having this moral obligation. It doesn’t explain why, if books will enrich future generations, we shouldn’t destroy them.