Maybe... but, they've already been in a zero growth state for over ~25yrs and it's been dropping for the past 10. Their economy has rumbled along and general society too. It is a question of interest, how much their relative lack of diversity has helped with the end to endless growth. Of course, as an export economy, the rest of the world opening up has helped massively, so when global population levels off in 50 yrs... say add another 50 for an economic levelling of sorts and then we'll see. Perhaps that should be 75 or 25 rather than 50, tis happening regardless.
We are continuously told about how the old people will be such a massive problem, but societies will find a way to muddle through. Public pensions etc. have only been a reliable thing for ~50yrs.