How many jobs do you have? Like as a professional in some sector of the economy, how many sectors do you personally occupy? Are you a farmer that also writes iOS software? No, obviously not. Whatever job you do is fixed and it contributes a finite amount of economic value. Since the population is now decreasing there are fewer people, which means if all the economic value from everyone is added up then the total contribution will be less than another population with more people.

It doesn't make sense to talk about zero-sum economics because economics is always a positive sum arrangement of work and specializations. The number of workers is the main limiting factor in any productive economy, every other measure is essentially an approximate proxy of that.

The financial sector on the other hand is indeed a zero-sum system. If someone is making money then someone else must be losing it because there is a finite supply of dollars in the economy at any given moment so if the supply is fixed then the monetary economy is a zero-sum game.

All of this is derivable from first principles but for some reason most people are constantly parroting some nonsense about wealth and zero-sums.