>The only reason it is a concern is that our economic system is built on infinite growth,

This is plainly, factually wrong. A sub-replacement fertility level isn't "the growth has stopped". It's "rapid shrinking has started, shrinking of the sort that causes even more shrinking". That's the concern.

> It's better it fail by people voluntarily having fewer kids

If they have fewer than 2.1 on average, that's not better. It's literally "you should die out and become extinct". That's what you're telling them.

>The solution is to fix the economic system

What you're pushing doesn't fix the economy, it makes it worse. When there aren't enough workers to maintain infrastructure, there will be rolling brownouts. It's food rationing because there aren't enough workers to keep agriculture going. It's "there are no more consumer goods because the factories were hollowed out"... which leads to "there is no retail and no retail jobs".

A shrinking, aging population wrecks an economy.

>Productivity is increasing all the time, and we're very likely on the verge of another huge productivity increase with LLMs and robotics.

"Robots will save us!" is juvenile fantasy. You're deluded.

>The idea that we need to constantly increase the number of humans

No one was talking about "increase the number of humans". Maybe you're just really bad at math. We're talking about "keep the number of humans steady, because shrinking locks us into a never-recovering vicious spiral of shrinking til we hit 0". Which is where we are now.