You keep insisting that most of the Highborne were killed. I do not believe it was even most, because the following:
Not quite. What you are referring to was from Elegy, before the Horde crossed the Falfarren River:
- The druids had learned that all the Highborne magi were gone. They had helped immeasurably in the early stages, setting aflame so many of the bridges and siege engines, but Sylvanas’s archers—and perhaps the Dark Lady herself—had homed in on them as targets to be eliminated as quickly as possible. And so, there were no magi to melt the ice that had once been a swollen river.
However, this was not literally all of the Alliance’s Highborne, just the ones that had shown up to help in the early stages, as in A Good War there were more Night Elf Mages later on outside of Astranaar:
- A group of night elves—most, but not all, had been druids or magi—had been hiding in the depths of this thicket. When the pack of Horde troops passed by, the kaldorei had unleashed arrows, magic, blades, and every other instrument of war, wounding almost the entire raiding party. All thirty of the Horde had been brought down within seconds. The druids had called upon nature to subdue most of them, and the magi had trapped a few in ice. Perhaps one or two had died quickly. The rest had been helpless—in pain, but alive.
And only then had the killing truly begun.
These Horde soldiers had not died in a flash of fire—they had burned slowly, in agony, screaming.
The night elves had done everything they could to prolong the horror, to maximize the pain.