I'm not sure your post title matches your content, so I'll try to answer as best I understand your question.

In a post-growth world investment wouldn't be a way of getting a financial return, it would be to fulfill a particular aim. I.e., you/society would invest in a wind farm because we need the energy out of it, not because we can sell the energy at a profit. Investment to make a profit is a basic function of a capitalist economy, and one that bakes growth into it's fundamentals.

So the word "investment" would mean something more like "we have an upfront cost to pay in order to get a societal benefit out of this thing we want to build/set up".

In order to get to a post-growth world, we need to undergo degrowth. Which means "investing" in the parts of the Global South which still need to grow, and "investing" in movements to try to make the Global North degrow.