Retirement and unemployment are basically the same in this case. Generally in the US, people do neither, they become disabled. I don't mean they literally suddenly need wheelchairs. I mean that when a large factory in a town closes down, within a year or two, the number of people on disability linked benefits goes up enormously. Then they die "deaths of dispair", suicides, opioid over dose etc. I know this sounds pretty fucking dramatic, but go look it up. Plus you get the likes of Trump elected...
Im all for productivity, but we are very very bad at making sure productivity growth doesn't mean shrinking incomes for the majority. So be careful welcoming the closure of the widget machine factory (or it getting more efficient and laying people off).
There is a rate of population decline that's consistent with economic growth. But its very small.
Three things generally grow economies: pop growth, productivity improvement and technological advance. If your pop growth is negative, you need the others to be big positives. Except that productivity growth is hard to achieve, hard to measure, and (as above) needs a lot of political management to achieve without leading to serious social issues. So you're reliant on technology. And technology isn't really moving much at the moment.
Japan and China have both suffered these combined effects for a while. Falling, aging populations. China has been fine because they can increase both the technological and productivity levels of its economy. It can move from doing things manually to semi automation (now basically done) and it can move from doing low productivity work (putting things together) to making entirely new products. Japan can do either.
Thats why China has managed huge growth (maybe not as good as they like to claim but still) over the last 30 years. And Japan has basically languished.
We are turning into Japan, not China.