Japan is modeled off of a capitalist imperialist economy, similar to the US. And as such, it will stagnate and requires imperialism and increasing levels of exploitation to maintain its economy. It does not have the tools or methodology to develop further without imperialism. And thus, it stagnates.

Meanwhile, China is socialist. It uses dialectical materialism to solve social issues and promote cohesion. And instead of profit, it will seek to optimize material conditions, and identify the barriers to doing so, creating a strategy for development. Through Marxian economics, it understands that productivity comes through cooperation and labour, and not profit. Optimization can be performed throughout the supply chain through central planning.

China runs under C—>M—>C, while Japan runs under M—>C—>M.

China does have problems but it also has the tools to solve them, instead of just sweeping it under the rug.