That's the narrative the media and arm-chair analysts love to repeat. There is nothing that makes this statement true. Where is the iron-law of nature that says a population must continually expand? Who will force Japan to import labor?
It is simply this. They will have a shrinking pool of workers and a growing retiree population that has to be supported. Imagine if half of our population aged 12-50 disappeared overnight. The results would be catastrophic and programs for the elderly would go into meltdown mode.
There’s no problem with a population shrinking a little, but at the meteoric rate that the Japanese average is shifting from young to old the situation is economically dangerous. With fewer workers, businesses will be forced to shut down should the government not make new labor available through immigration. Although Japan has a very automated economy compared to other countries, they do still require lots of workers.
If there was no problem with it, they wouldn’t be spending so much time and effort trying to reverse it. Nobody there wants to import workers.