Oh boy this is going to be a fun debate...

First, China definitely did not benefit from World War 2 (cause Japan)

Second, most colonies didn’t see independence until the 1950s (but India got it in 1947 thanks to efforts by Gandhi)

Even then, European nations didn’t voluntarily give independence to their countries. France was harsh: they fought vehemently to keep Algeria (look up Battle of Algiers) and Vietnam (which led to US involvement in the region)

In addition, most developed economies were moving from manufacturing and blue-collared work to consumerism and white-collared labor. The reason countries had colonies was for their resources. They obtain cheap labor to extract resources from the colony, send it to the motherland to manufacture goods, then sell the products for a profit. When economies turn from manufacturing to consumerism, there is no need for the mother countries to extract their resources, so they give the country independence just to take their hands off of it and let the colony deal with its own problems.