According to the 2019 World Happiness Report, Japan ranked 58th out of 156 countries in terms of how happy its people are.
Japan has recently changed its law to let in more foreign workforce, mainly from Asia. They come to Japan because here they can earn on average more than five times they can earn yearly in their own countries, while knowing full well that they have to work like slaves, doing jobs that few Japanese want to do. The suicide rate among those young Asians coming to Japan to work is quite high, like 30%, from what I gather from the media. I feel sorry for them.
Moving to a different country would entail a lot of issues. One major concern would be, understandably, if you can fit in.
I think Japan's health insurance system is one of the best around the world, hence the highest longevity around the world.
Happiness is a subjective thing, that's for sure but I think those elderly Japanese who consider moving somewhere else to spend their life after retirement should take those objective criteria into consideration before taking the plunge.