The tie-in books also say that the nelves began evacuating in advance. They knew the Horde was coming and unlike the in-game questline where they barely put up a fight, they were engaging in all manner of skirmishes with the Horde in order to buy time for those fleeing. So chances are that a decent amount of nelves actually did survive.

Also in the novellas Tyrande actually does something beyond showing up to just grab Malfurion and fuck off to Stormwind.

It's just a shame that they fail to convey any of this in-game.