this is probably my inner uptopian leftist speaking but -
robots. And the fact that a large amount of the work doesn't actually need to get done.
So long we keep enough skilled tradesmen and farmers, which is not that many with increases in automation, we'll still have enough food, shelter and clothing. Being able to spread that work out amongst more people because bullshit jobs aren't a thing anymore will also probably help, as will changes in lifestyle that enable people to stay healthy longer.
We really don't need a new H&M collection every two weeks, or a new sport jersey every two seconds, or to keep making more and more cars/washing machines/TVs... most manufactured goods (including clothing) can last a while, especially in a world where planned obselence is no longer the norm. If population stabilizes, need for construction drops as well. Food/agriculture is already very automated, I'm sure we can continue to find 3% of the population to keep us fed.
I believe most of the work currently done on earth contributes very little to human happiness, and I hope/want to believe that as our ability to do work diminishes with an aging population, the useless stuff will be the first to go.
silly example, but with GPT-3/ChatGPT, we can now fire all the influencers and have them go toil as construction workers :P
If the birthrate drops, neither you nor I nor the kids alive today will get a "retirement". It'll become a quaint little thing we once could afford.
Not so bad if all your work is tapping into a laptop. It's bloody murder for anyone that has to stand and lift and DO for a living.