Wouldn't a decline be good? Or is the theory that we can only do collapse, not decline?

Depends how it's done I guess. France has a decent population tower, for lack of a better expression:

https://www.populationpyramid.net/france/2026/

If they keep it up, their population will remain stable up to the 22nd century. There will be no further noticeable ageing and thus no threat to anyone's retirement.

Decline fails to uphold the prevailing world economic ideology. That's the real problem with population decline the way I see it. We will need to adjust in ways that don't support the idea of "unlimited growth".

Line must go up. Part of me wonders if the economic ideology might be what causes more than just a gradual decline.

> Wake up grandpa

Just imagine how many more grandpas and great grandpas will be around for you to shout this at.

The problem with the population isn't (yet? only?) the absolute number of individuals but the age distribution. Most societies rely on young people to support themselves and the old, and this only works if there's a good balance between the numbers of each. Living longer isn't guaranteed to offer a longer productive period. We might just end up with more old people in need of support and not necessarily more children. This is already a major problem today in most developed countries.

There are probably so many more factors to consider. Like the psychological impact of living substantially longer. Even things like that crappy boss who won't retire from another 60 years, the dictator who won't die for another hundred years, or any number of the many society destroying tech CEOs getting to entrench themselves for decades more.

"on the cusp of beginning to"

This will happen no earlier than the 2080s. The world population is expected to increase every year until then. We will add nearly 2 billion people in that time. Consider that the entire world population was less than 1 billion for all but the last 221 years.

I continue to not worry.