Japan is of course more prosperous than China, and long has been. But Japan also has pervasive use of syllabic writing (the katakana and hiragana syllabaries that are each capable of exhaustively writing anything a speaker of Japanese can speak, with very few written characters).

Prosperity may actually have more to do with effectiveness in spreading a standard national spoken language. The paradigmatic example here is Taiwan. When the Nationalist (KMT) regime from China fled to Taiwan (formerly occupied by Japan) after World War II, hardly anyone on Taiwan could speak Mandarin. Mandarin is about as cognate with Taiwanese (or Hakka, the other main language of Taiwan) as English is with German, or arguably even less cognate. But today anyone my age (fifty) or younger in Taiwan is conversant in Mandarin, even though the great majority of families living in Taiwan have older relatives who didn't speak that language at all. Prosperity brings telephone conversations and radio and TV broadcasting and travel and other human activities that converge language usage to a common standard. By contrast, the population of (mainland) China has long included a sizeable number of people whose native language is within the dialect category of 北方官話 (Mandarin) but that initial seed value hasn't resulted in a very impressive increase in the percentage of Mandarin speakers in China during the post-war years. That's a sign of the stark difference in prosperity between Taiwan and China.