Even "curing" the Forsaken would mean that the Horde loses a race and that wouldn't go down well with the Forsaken fans. If Blizzard won't cure them, they surely won't destroy them.
I can imagine Blizzard having the idea of curing them at one point of time to create a mindblowing plot, but that goes down the drain due to the MMO genre and the whine that would come with it.