I pondered a little about the Scourge threat in the Wotlk and came to the conclusion that the Warcraft community greatly overestimates the threat of the undead army.
People really overestimate the Scourge and its threat to Azeroth very, very much. Arthas just attacked at the right moment. No one from the powerful of the world considered the Lich King such an important threat to be distracted from other matters. In fact, the red, green, and bronze teamed up to stop the blue dragons, and not undead (although there are a couple of dragon quests in the game just to oppose the Scourge), and the wild gods did not have to be resurrected urgently to stop the terrible Lich King, as in Cataclysm, because the Nightmare was much more dangerous than the Scourge.
The Alliance and the Horde are weakened by the war in Outland, the red dragons (and some others) fight the blue, the greens along with the wild gods, the ancients and Malfurion fight the Nightmare, the bronze fight the infinity dragons, the titan-forged and Keepers subdued by Yogg-Saron. In fact, even when the Lich King resurrected Galakrond, Alexstrasza was worried about mortals and said that if Galakrond was resurrected, mortals would have problems. She felt no threat to herself personally and decided to continue to fight Malygos.
The Chronicles said that the defeat of Kel'thuzad greatly interfered with the strategy of the Lich King, but fortunately the Alliance and Horde were distracted by Malygos. Then he was nearly killed at the Wrathgate, but the Legion (well, the servants of the Legion) saved him and Alliance and Horde had to be distracted by stopping Varimathras, whose activities Sargeras personally monitored (maybe he wanted to make a new invasion of Azeroth?) and then the Alliance and the Horde went to fight with Yogg-Saron.
Even with all these parallel wars, the Alliance and the Horde defeated the Scourge (even if the Lich King allowed it, but if he could destroy the weakened Alliance and the Horde, he would have done it. Conclusion? Even after all these wars, the Scourge could not to defeat the Alliance and the Horde).
The words of Terenas is also only his personal opinion, he has no idea about Deathwing, the Old Gods, Aspects, Wild Gods, Keepers. Yes, even in the Chronicles it is written that Terenas said that the Scourge would cause irreparable damage to Azeroth (not destroy it).
Well, I would like to remind you that in the pre-patch of the Shadowlands, the Scourge will remain without control and will attack the Alliance and the Horde. But we all know that the Scourge will not be able to destroy them, simply because it is an MMO genre and the Alliance and Horde must always exist. That is, even weakened by constant wars with the Legion, the Old Gods and with each other, the Alliance and the Horde will reject the Scourge.
Ha, to some extent Lich King even helped Azeroth, because he lured the Alliance and Horde to Northrend, where they defeated the Old God, who otherwise could have been hiding even longer. Ironically, during the Wrath of the Lich King, the Lich King and his Scourge were the smallest threats to the world.What do you think?