I recently moved back to the US from Japan after living there for a year. My poor Japanese made life very difficult. Easy things like calling a restaurant for a reservation or visiting the ward office was always a major challenge. I second your point that Japanese is always useful regardless of official policies. What do you think Japan should do to encourage Japanese language learning among immigrants?
It really boggles my mind how many immigrants to a place (because it doesn't just happen in Japan) are fine not trying to learn the language, especially if the place doesn't even understand the languages you know. You'd think living in such a place would be enough encouragement (it certainly would be for me), but I keep seeing stories about immigrants in several places not bothering to learn a common language of where they live.
The government has been making efforts, such as trying to improve the training and certification of Japanese language teachers, making Japanese language ability a condition (or at least an advantage) for getting certain types of visas, and offering language support to immigrant children in public schools. They are also trying to promote the use of simplified Japanese—avoiding difficult vocabulary and indicating the readings of all kanji—in documents and services aimed at the general public, something that would help not only immigrants but also Japanese with lower literacy skills. I’m sure much more could be done, though.