Neither am I. But, the problems that are, and will be created by falling birth rates and a smaller population overall, are real too. That can't be denied.
Now, are they unsurmountable? No. Should they be talked about? Yes. Locally, the high/middle school was over built just a few years ago. It's already clear it's far bigger than we need now, or are likely to need for many, many, many years - maybe ever. (Our 'average class' size has fallen from ~200 to barely 120-30, at best... In just a few years. With no signs of ever improving.)
When does it end, though? How many human beings can this earth support? At what cost of natural resources, flora/fauna, environmental harm?
AI is increasingly taking over jobs that previously required humans. How many humans are actually needed to support current populations? How many of these now-displaced humans will enjoy enough resources to sustain themselves while increasing humanity's knowledge base, artistic pursuits and general intellectual/emotional enlightenment?
These are not rhetorical questions. I don't know the answer to these things. It seems to me that increased population for the sake of population growth itself is not correct.
On situations like Japan, there seems to be a trend of income and purchasing power with having children. If people want children, but can't afford them... They won't have them.
You can see a higher percentage of wealthy people having children vs a lower percentage of working poor people having children everywhere. The only way to increase childbirths naturally is to make working affordable to support them.