> Presumably the spoken Japanese language predates the writing system, no? Or if not, I would imagine that a couple hundred years ago, a significant portion of the population couldn't read Japanese but could speak it.

You're correct. Prior to the industrial era, most societies had very low literacy rates. But nearly everyone could speak, and many people who were illiterate could even excel at speech and language related tasks.

Among the worlds most grammatically complex languages are the Inuit languages of the far north areas. These languages were completely unwritten until very recently.