If the world’s population stops growing, then a lot of economic assumptions about economic growth, investing, debt, etc are no longer valid.
For example, there may be a surplus in funds for investment by retirees, driving rates for safe assets to near 0%. Asset prices would probably go down globally while wages increase, until the world gets to steady state. Meanwhile borrowing money for “risky” investments or real estate would go up significantly because no one would expect asset price increases.
State state, of course would be a big ?. You could just be for setting up for a massive decline in population as a result of people not having kids because the economic situation will be worse.