Eldar's ancient enemy will be chaos. To cite DoW "Vile servants of Chaos approach, the great enemy" "Our ancient war against chaos continues"
The whole eldar way of life (ie the path system) is a direct response to chaos rather than the wars against the necrontyr.
To the eldar chaos is the great enemy that must be bested or the universe will die. Its cited in pretty much every source that they have done more than any other race to stop it
The Necrons Wars haven't been forgotten but it seems the eldar response to them is watch over the tomb worlds to try to stop them coming online, but otherwise use non eldar to fight them large scale. They know they (Maybe no one) can't fight the necron on an even keel should they all awaken