It was said that Fire Lord Ozai would use Sozin's Comet to win the Hundred Year War. If that happened, even the Avatar would never restore balance.
If Ozai were defeated after he used the comet's power to wreck the Earth Kingdom, it would mean forever imbalance. Ozai would leave his mark on the Earth Kingdom, buildings wrecked, people killed, and it would take a long time to repair the damage caused by Sozin's Comet's misuse by Ozai's firebending, and the hearts of the people, who entrusted the Avatar to look after them.