Actually, they most likely couldnt. Sure they could break through the walls eventually, but they didnt have trolls with them... Plus even if they destroyed Lordaeran, the Alliance Army can still squash the horde at this point at full force. Horde kept sending and sending to where they would "eventually" overwhelm and destroy there enemy, they didnt have a constant supply, they had 1 giant force that slowly was destroyed. Also Stormreavers wasnt a big loss besides lack of warlocks, Stormreavers was the smallest clan of only warlocks. Its possible they would have taken Lordaeran, but that wouldnt stop the Alliance Army. All the horde had left with them at the time was i think a few dragons, few ogres, and lot of orcs, thats really all. Trolls stayed at Quel'thalas, thats the only reason they joined the horde was so they can attack quel'thalas