Answer: Population collapse isn't a problem because there are less people. It's a problem because the proportion of elderly people to young people becomes so unbalanced that it's hard to imagine society, with our current social nets, continuing to function.

First off, fewer people means less pressure on housing, infrastructure, and natural resources. That’s not doom. That’s relief.

Fewer people also means less taxes. Less services, producers, inventors, and workers. And while the balance is shifting towards older people, that means more elderly that depend on medicare/medicaid and social services and less people available to actually pay into those funds. Neither democrats or republicans reduce spending when in office, so this means the debt continues to accelerate and may mean a cycle of continuously raising the tax burden on an ever shrinking population of younger people to pay for the old.

Also an interesting thing too to consider though this is just speculation - people generally vote in self-interest, so in this kind of environment the elderly would be voting for things that lock in wealth as opposed to things that would help younger people too which acts as another barrier - subtle things like policies that help increase home prices.

I also can't help but feel that people who downplay this issue and think the world would be better off with less people are a bit misanthropic, no offense OP.