I don't see how you can count the benefit of all donations in a chain within the donation of the first donor.

If you're trying to compare actions, you should say "how will the world be if I do A instead of B". If you think the chain truly wouldn't have happened if you hadn't decided to donate your kidney then the benefit of all of those people receiving kidneys happens in the world where you donate, and not in the world where you keep your kidney.