This is , overall, looking on the trends. If Japan faces an economic decline and it comes something like the Dark Ages, people will have far more kids in this climate. It also doesn't take into account if more than likely , in the event they become third world in the next few centuries, they will have more children.
It also neglects to mention if Chinese immigrants or Korean immigrants, both of which have a ton of children , would fill the gap and make the Yamato people a minority while the Ainu go extinct due to assimilation. Then again, Koreans have a ton of common ground genetics and lingual wise with the Japanese so it's not like it'd be a total loss.