Human population would be why we don't want a high birth rate.
The parent comment was asking why we do want a high birth rate. Increase in economic output from population growth in a consumer based economy is a reason that economists would give and that politicians would use to support the policies promoting a higher birth rate.
I think we probably could eliminate most policies promoting population growth. Of course some of those could meet some ethical concerns. If we start talking about policies to limit population growth, that could get even trickier.
For one, the word "above" is unfounded. At or slightly below works too. You can have a consumption-based economy where the population drops 1% per decade just fine.
But more than that, even the least capitalist economy is going to have trouble if the ratio of young healthy workers drops too low.
I agree it's mostly about the ratio of workers. It can be below for relatively short periods (a few generations maybe). But if it stays below for long periods of time, you will run into some structural issues. Like the young workers you mentioned, tax burdens, some consumer durable industries, and possibly housing/real estate.
A slow rate of decrease doesn't really run into issues. To use the number of 1% per decade again, if we kept that up for 300 years we'd have no worker shortages as we slowly crept down to a "mere" 240 million people. (For reference, the population in 1950 was 152 million.)
Don't you still run into labor force participation rate issues due partly to wealth accumulation when shrinking the family size?
For example, if you receive your parents inheritance at age 50, you might be inclined to retire early. Even if you have to split it with one sibling, that could still be enough for early retirement depending on the specifics of the assets.
Even between relatively extreme examples of "every family has 2 children" and "every family has 3 children", the difference in inheritance isn't that big. So I wouldn't expect much more early retirement than if there was mild population growth.
