But they're still - for now - on a much better path than the West.

They really aren’t. They were among the first countries where the TFR dropped below replacement. The fact that they didn’t have replacement immigration up until now doesn’t give them that much of an advantage, as in the end it’s all a numbers game. The median Japanese is 49 years old. This means that roughly half of Japan’s population is irrelevant in the grand scheme of things and it’s only going to get worse.