Yes, they can! And IMHO they should. But that's a very controversial suggestion these days...

IMHO the largest problem humanity will face in the next 100 years might be decreased fertility rates. Unless our robot overlords solve that first.

Why would the human population shrinking to, optimistically, say, 3 billion people be a problem? Looking at our productivity, and unemployment rates, we have the labour, and the productivity to assist the elderly.

Because of the dependency ratio. An inverted age pyramid would wreak havoc in social welfare systems, unless you were able to triple or quadruple labor productivity.

Today we barely have the labor and productivity to assist the elderly, imagine if the situation worsens dramatically.

Sure, there's always the chance that AI and robotics and singularity will change everything and scarcity will be over and so one, but we shouldn't bet our chips on that.

We have already quadrupled labour productivity, compared to the 60s. Including agriculture, we have multiplied it by more then a factor of ten, compared to the start of the 20th century.

We barely have the labor and productivity to assist the elderly for the same reason we barely have the labor and productivity to fix potholes on my street, or keep human waste out of a San Francisco alley (All while we sit at 7% unemployment.) We don't care to.

And how certain are you that we'll do the same in the future?

I don't have to be. Since we're far more efficient now then we were half a century ago, even if workforce participation goes down, we would still produce a lot more then we did in the 1960s. Relatively speaking, regressing to that level of consumption would be a relatively minor lifestyle adjustment. Let's also not forget that the elderly can, and do useful work.

Compared to the problems of overpopulation, this doesn't even register as a crisis.

Sorry, but that doesn't make sense to me. Look at today's highly unbalanced budgets around the world, and heavily deficitary pension systems. How would that work if you decreased revenues by 10% and increased payouts by 70%?

You're missing out on the dependency ratio. Take the Euro area, for example, today, for every 2 people in work age, there is 1 person being sustained (youth or pensioners), by 2060, for every 4 people in work age, 3 will have to be sustained, an increase of 50%, and it will be far more expensive than today, due to health care costs for the elderly.

Work age population will fall from 335M people today to 300M in 2060, while elderly will increase from 90M to 150M and youths will stay roughly the same.

So 35M fewer people paying taxes, and 60M more people needing support.

This goes right back to my earlier point with respect to what our society prioritises.

There are more empty homes in America then there are homeless people, but that doesn't mean that the richest country in the world cannot afford to have a roof over its head.

This is not even considering the easy way out, which is more immigration.