Why does a growing economy require a growing population? There's not a very strong correlation between population and GDP per capita - the US has the highest GDP and there's several countries with significantly larger populations.
To have higher GPD US atleast doesn't require more population.
Here is why:
1. American companies sell outside US so we can benefit from selling to India, china and Europe.
2.Even if we aren't exporting to some countries, there is still demand for our currency as they can use it to trade for other things like petroleum
4. Even if we've no petrol or exports, rich people and companies from other countries invest in our treasury bills, bonds and stock market.
Look at how many companies of different countries even like the ones from UK, India and China invest into US market because of its safety
A growing economy requires, in the large, an increasing labor pool, but that labor can be human or machine-amplified human. If the amplification rate is growing fast enough, we can grow the economy with a shrinking headcount.
(The other side of the equation---consumption---also balances out; people with more leisure time consume more).
-As people grow old, they are economically dependent upon people younger than them.
-A growing population goes hand-in-hand with a growing economy. This tends to bring prosperity to many people.
And as a bit of a tangent: The argument that a shrinking population is good for the planet and is therefore better than a growing population feels to me like an extremely pessimistic argument, and the people that I personally know that embrace it have a much less human-centric moral intuition than I do. I put a lot lot lot more worth on human happiness than that of animals, and to me it seems that a cultural disconnect from the natural act of procreation implies an unhappiness that is rooted in how cultures are run nowadays.