There's a hole in the lore where we don't exactly know the mechanics of Void Lord manifestation the way Sargeras understands it. All we know is Sargeras has come to the conclusion that wiping all life from the universe and reordering it in a different way from the start will fix the problem. If I had to guess, flesh is the problem. The fact that stone and metal can turn into flesh is another problem. So maybe he will rebuild the universe without stone and metal?