And yet if Buffy had gutted Dawn like a fish and tossed her into the portal, there would still be those who complained....
Dying to save the world is avoiding a choice if you can kill your sister, or let the world be destroyed...I have to say that logic does not work for me! I also assume that if the portal had not disappeared then Dawn would have jumped as she was prepared to do. This wasn't just Buffy's choice.