Your idea that Japan's lost decade was caused or extended by a low birth rate doesn't sound right to me... and the first two results in a search engine don't support your claims. It would also not appear to be provable or disprovable. The lost decade certainly wasn't caused by demographics, and there's no alternative universe Japan that encountered those same economic conditions but many more babies were made.

For detractors...

Social Security in the US is one example. It literally depends on having a larger input base than the output.

The expectation of GDP growth is another, but without population growth, it's unlikely to be realized perpetually. I'm fine with that, but our over-leveraged financial system isn't.

Exponential growth isn't going to be sustainable forever. The economic models need to be updated to match reality, instead of the tail wagging the dog.

> A lot of our economic machinery more or less bakes in some big assumptions about the size of the workforce not declining

Do it like software: if the implementation doesn't match reality, fixing the implementation is easier than trying to fix reality to match the broken assumptions

1. There are a lot of goods where supply is naturally limited. Nice holiday destinations, for example. Good housing in desirable locations is another. Space on public roads. This implies that there is some population size where some notion of real wealth per capita is maximized. That population size may be much lower than 7 billion.

2. Economic growth can be driven by increases in productivity. Automation can massively boost productivity. But at a certain amount of supply of labour, it is cheaper to employ a human than automate the task, even though the automation technology for that task is within reach. So a reduction in supply of manual labour can actually make the economy more automated and efficient.

We actually just need a few more smart and bold ones