Yes, Dawn should've died. It would've made a far better "ending" if she'd volunteered to do so, and Buffy tearfully agreed. Their last conversation would've shown Dawn's bravery, and Buffy's reluctant willingness to sacrifice her sister to save the world.
Now, if THAT didn't happen, perhaps Connor could've made an appearance...and they both jumped together.
Or, if I'd been a guest star, I would've distracted Buffy by telling her that Angel, Willow, and Spike were being held hostage by a sadistic demon, who specializes in killing vampires and gingers. While she was gone, I'D throw Dawn into the portal. Then I would run like hell, get home, and have some buttered popcorn. 😁