Do murder really gets solved if some random decides to write a suicide notes and confess in a text?
There isn't any new evidence, and existing evidence wasn't enough to lead to a suspect so it'd be no different if the neighbor confessed right?
I don't know enough about criminal investigations in Japan but at the very least in the US, all the evidence the Police had would've pointed to the Husband as well. The same problem would've occurred though, you've got motive but no actual evidence that directly connects the husband to the crime.
Also, not sure if I'm misinterpreting your comment or if you didn't catch this so I apologies ahead of time, but the "random" that "wrote" the suicide note
was the neighbor. You make a solid point though!
I'd have to re-read this myself but I seem to recall the detective mentioning noise complaints from a neighbor in his original investigations. If so, then it's the fault of the detective and the investigation unit for not exploring that as a possible motive either.
For what it's worth, motive doesn't have to be something big like "husband cheating on wife doesn't want wife and family to find out." It can be, and very often is, something so small just like a noise complaint or "I didn't like the way that person looked/talked to me."