Japan's population decline isn't really about cost of living so much as it is about poor quality of life/a stiflingly conservative culture. Terrible work/life balance, women still largely being expected to quit work when they have kids (and it being assumed that women will have kids, leading to discrimination in the workplace and limited career promotion opportunities), men still largely being expected to be salarymen who are forced to prioritize their company and social expectations at work above all else, etc.