Capitalism requires more people in order for companies to make profits and stock prices to go up. Socialism requires more people to pay for the safety net. Welfare State Capitalism (which is what we got) needs it doubly.
Either there are more consumers or each consumer buys more. But consumers are now so far in debt that there's no hope there either.
the World population keeps going up
the ism doesn’t matter
the world is not in danger of a declining population from any metric we’ve seen comparing births and deaths
plus these very same fearporners on this are the same people who are population reductionists or zero pop growth people, to begin with