Bad (and the few ok) population decline arguments

The population doomers/natalists are incredibly annoying, only slightly less illogical than climate denialists.

There are several reasons for this, and the most important one is that they're Ponzi-schemers in denial of their Ponzi-ism. The essence of the scheme is that it is easier to plan for future economic problems by assuming that there will always be a larger working age population than there was before, forever. This is, in fact, a problem with national retirement systems that pay out more than they can make from contributions plus interest. Population slowdown or reductions expose this problem earlier, but it still would have happened.

So, aside from playing along Ponzi-scheme pension planning, is there a reason to really be concerned about population decreases? Sure, in theory: 

Under all reasonable scenarios for population decline combined with minimal economic growth, future retirees will draw benefits from an economy that is much richer than the  present. However, that richer economy will be moderately less rich than it would be if population were unchanged. It would be significantly, if temporarily, less rich than Ponzi-scheme proposals that assume every younger generation will be bigger than older generations, forever.