I rather think the Chinese will have something to say about this at some point in the future.
China is experiencing its own population collapse, which is even worse in scale than Japan.
I have several friends that have lived in Japan for years.
True. But they do make it EXTREMELY difficult to live there if you are a foreign national. I worked for a company and ended up living in Japan for nearly four years in the early 2000's, and thankfully my company handled most of the bureaucracy and paperwork. It was so onerous and such a full-time job that the Japanese women hired to shepherd me through the process charged the company close to $100k USD for the four years I was there.
The general consensus is it would be impossible for Joe Blow off the street to just decide one day he wanted to become a full-time resident in Japan. For one thing the vast majority of Japanese will not rent to a foreigner (which is perfectly legal in Japan) and those that do often want payment up front. And by "up front" I mean if you are planning to live there four years, they want the ENTIRE four years rent up front. Also, if you are thinking about buying a home, there are all kinds of additional hoops to jump through, most Japanese banks won't loan money to foreign residence, so you pretty much have to cough up the purchase price in cash.
Then even simple things like getting utilities in your name are virtually impossible as a foreigner, I ended up having to get mine in my company's name, then they would bill me once the utility companies billed them.
There was even one time my cell phone died, and I tried to get a replacement. I quickly found out that cell phone carriers in Japan won't even deal with foreign residents, even if you are willing to pay up front. Again, I ended up having to get a phone in my company's name, then getting my own when I returned to the states.