It's funny, still today it has like 4x the GDP per capita of China and yet hearing it from these propagandists^Weconomists you'd think China is some kind of futuristic wonderland where everything turns to gold and Japan is a crumbling wasteland (or soon will be.... aaaany day now, for the past 30 years).
Japan's got issues, every country does. You think the US or Europe don't face serious economic problems in the next 40 years? As populations around the world peak and resource extraction and primary production peak, cheap fossil fuels are phased out, everybody's chickens will be coming home to roost. That's not a Japan problem, that's everyone's problem. Rich countries like Japan have a lot of room to cut expenditure and raise revenue. They don't because it's not popular, but they can and will (have to).
It does not mean that Japan is doomed, but rather that some policy will have to be implemented and some large change to the structure of the economy will have to happen in due time.
The thing with demographic problems is that they are background noise for a very long time and then suddenly become a much bigger problem. You can verify this yourself by playing around with some simple math such as having a cohort of 100 people at t0, set an average expectancy, and seeing the effects of different birthrates as time goes on.