Why did the Polos fear Kublai Khans death?

Kublai Khan was the grandson of Genghis Khan and the founder of the Yuan Dynasty in 13th-century China. He was the first Mongol to rule over China when he conquered the Song Dynasty of southern China in 1279. Kublai (also spelled Kubla or Khubilai) relegated his Chinese subjects to the lowest class of society and even appointed foreigners, such as Venetian explorer Marco Polo, to important positions over Chinese officials. After failed expeditions against Japan and Java, his Mongol dynasty declined toward the end of his reign, and was completely overthrown by the Chinese after his death.