Japan is squeezing more and more out of what it has to work with. When their population does finally start to physically shrink, then I guess they'll have to increase the automation to stay ahead of death. Or, increase immigration. Or, start having more Japanese babies. Or, die.
What's 20 years when we're talking about the larger picture? It's not even a drop in the bucket. Nothing is going to disappear overnight, and certainly not an entire nation like Japan, because we have however many years/decades/centuries of history, momentum, and complexity built in. If we had zero population change right now, so one birth for every death, we'd still be able to grow economically because so many people on this planet don't have what so many other people do.
However, we don't have a lot of experience with populations that actually decline. Every institution we've built is based on the idea of more people. When a business doesn't have enough customers, it goes out of business. When a government doesn't have a big enough tax base, things start to not get paid for. Everything is still growing, even if just a little bit, in one form or another. When/If there are physically fewer tax payers and consumers, then what? Japan can get away with it for now because people around the world buy their stuff. We're in a weird place where we have a more global society, yet regional governments. We try to balance that, but it doesn't quite work.
Like anything else, it's adapt or die. If the Japanese population gets older, but they increase immigration, it won't be the Japan that we know, it'll just be a different Japan, which is fine. If they stay on the path they're on, it'll still end up a different Japan. Which goes back to nothing staying the same forever.
The life cycle of a society isn't that much different than the life cycle of a human being. Start out small, grow, mature, and then death eventually wins. Japan is in the mature phase. You can get some health care to prolong things a bit, but it gets tougher and tougher each time.
The US is an odd case. If you look at the largest populations on the planet, they're all from developing nations, except one. We have the population of a developing nation, but the economy of a developed nation. There's really no other country quite like that, which is why the US has it's own unique set of issues.
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