I don't agree with a decreasing population for Earth. I think we should have trillions of people on Earth eventually, and we should turn Earth into a high tech Ecumenopolis no matter who the people are.
My point really is that a decreasing population on earth just isn't a serious problem. It's a problem for an economic system that requires perpetual growth, but that system is dumb and I don't think it should survive. Even if you somehow believe it is desirable for perpetual growth to continue, it's not hard to imagine reaching the point where automation allows for that despite a declining population (and where people simply consume more because they have more resources).
Sure, you might say "think of all the great scientists and artists who won't be born" but think of all those who are born today and end up working in sweatshops or never getting the education they need. Our current society is shockingly bad at utilizing human intelligence, I don't see any reason why fewer people would slow the pace of human innovation, particularly since at that point machines would likely be augmenting human intelligence to an even greater degree than they do now.
For most of human history, the global population was measured in the low millions. Even a thousand years ago, it was only around one billion. It would take a very, very long time for population decline to become a threat to human survival, and again, I would like to think that by that point some kind of solution would have been found.
I don't care about a humanity that looks nothing like us, I just want humanity to remain a thing. How are we going to terraform Mars with 3 billion people plus have colonies in the solar system including a Dyson Swarm on the sun without it.
Robots.
Humans were never going to build a dyson swarm or terraform Mars, we're extremely bad at stuff like that because we require all these weird resources to maintain and because, unlike robots, we're not expendable. It takes a long time and a lot of work to make a human. Make a robot able to replicate itself and they can grow exponentially until they literally run out of resources.
The problem we have is a mindset of less is good.
I think it's more that we have hit the point of being able to question
why more is good.
For all of human history, there was a constant need to expand. To fill the available space and produce more people to exploit more of the resources and do more of the work. Now we're running out of space and resources, and the work has become so efficient that most of us aren't economically necessary any more. More so than at any point in history, our lives have become less about needs and more about wants. We
needed to expand and make more humans, but did we ever want to do that? Probably not, it was pretty miserable.