>>mufti_+k4
Because Japan's population problem isn't a wealthy nation problem it's a population collapse problem. It's not just that birth rates have gone down because the average Japanese person has become wealthier or more educated. It's also that the lost decades made raising more kids unfeasible for a generation and because of that there's now a large elder population that must be taken care of by a smaler population. Immigration at least ameliorates some of that problem, plus the economy is doing better which means people will have more time and wealth to raise kids.
Even if you can't reverse the population decline you can at least not make it a dramatic collapse and go through all the human suffering that entails.