I think you are misunderstanding the intention of the book. When you see a book titled "Japanese for English speakers", what it means is that it builds on your

I think you are misunderstanding the intention of the book. When you see a book titled "Japanese for English speakers", what it means is that it builds on your knowledge of English to teach you Japanese, not that its goal is that you replace your usage of English with Japanese. Certainly, the book will not include a justification of how you can use Japanese to speak in London and New York as you can do with English, and probably it won't even bother explaining you why learning Japanese may be a good idea. And, yes, you probably could learn Chinese instead, but that is not the point.