Reducing earth's population and zero-sum thinking

Some environmentalists want to drastically reduce the human population by convincing people to stop having kids. Megan McArdle points out that this would harm the elderly because with fewer young people to do the work, there would be vastly fewer resources for the elderly to consume.

I suspect that many environmentalists' zero sum view of economics causes them to believe that the elderly would be better off if there were fewer young people. If you think that the Earth has a fixed amount of wealth, then indeed with fewer people alive there would be more for everyone to consume.

But most wealth comes from the labors of healthy humans. If the human population falls, the amount of wealth available to everyone alive would also fall, probably by far more (on a percentage basis) than the fall in the population.