I don't think the popularity or her boobs will play a role in the end. If she goes Lich King style she will have the same fate as the Lich King. Remember that the Lich King had the same fanbase behind him. I was one of them. That however didn't stopped Blizzard to kill him. Thing is that there will be an Army of Light and the Forsaken will take part in it. However I think that Sylvannas has to be replaced in order for that to happen. As long as the Forsaken have the attitude of the Scourge(hate against all living and turning them into undead) I don't think they will be part of that army.