I don't think China would stay divided in the long run--China had been unified for quite a long time before that. There is some thoroughly massive cultural inertia here.
Not actually true.
China was unified under the Han Dynasty (206 BCE-220) and the Tang Dynasty (618-907). But note the 400 year gap between.
From 907 to 960, China was again disunited.
The Sung re-established unity in 960, but in 1127 Jurchen invaders seized the capital and all northern China, as the Chin Dynasty. The division continued until 1234, when the Mongols conquered the Chin. The Mongols conquered the Sung in 1279.
So - as of 1234, when the Mongols invaded, China had been united for 882 of the previous 1,430 years - and disunited for 548 years, including 160 of the last 327 years.
Since the Mongol conquest of the Sung, China has been continuously united, and imperial unity of China has become a sacred principle it Chinese culture.
But that's now - after over 600 years of continuous unity. In 1234, it was not so well established; and it is possible that a division of China could have persisted and become permanent.