This was actually talked about in the Before the Storm book. That maybe the Forsaken as a race should die out. You got your second chance and maybe that should be enough. Especially now that we know the Afterlife is an actual place.
Having said that, such a scenario would only ever occur at the end of Warcraft proper. In the mean time if Blizzard wanted a story reason to “have new forsaken”. That is actually quite easy, move the Forsaken to Northrend. We know the Scourge will always be a minor threat, meaning there will always be new freed Scourge.