These kinds of simple analysis suffer from what's called lurking variables. Like Anna said, Japanese diets are also high in other beneficial dietary components, so it's not obvious that their longevity is due to salt, or even that it is due to diet. Also, I don't know how many nutritionists and primary health practitioners out there advise people to eat below the recommended daily value of sodium without cause.
Reduced sodium is helpful for those who have medical conditions which already lead to high blood pressure.
There needs to be a reason to mitigate your dietary choices.