How did the End Times happen when The Old World shows Grand Cathay could have easily defeated all the forces of Chaos, including Skaven and Beastmen?

Miao Ying could have killed Archaon without breaking a sweat and she's just one of many dragon children. Shugengan wizard-warriors are far more powerful than chaos lords. Grand Cathay outnumbers the Skaven too, with Wei-Jin canonically the most populated city in the entire setting.