I was just rewatching and I saw another way to view it.

Dawn was made from Buffy. She's Buffy's essence, only without the Slayerdom. When Buffy dives to her death, she'll continue to live in/as Dawn, but her Slayer journey will have been fulfilled. And she's ensured that she/Dawn will grow to be a good and useful person anyway. Dawn's willingness to kill herself is something that the original 14-year-old Buffy wouldn't have dreamed of doing.