I wish I had got to this post earlier.. so much awful illogical information in the first page.
Couple of key things:
1 - The issue with depopulation is not less people = bad. It is less young people = bad. When you end up with 20, 30, 40, 100 retired elderly people to 1 young workers, your society fails. No ifs or buts, unless there is an AI miracle, you cannot support a society without capable workers. No society = no modern life, no food, wars, system collapse etc.
So yes, in theory if you could go from 8bn people to 4bn people, keeping the ratio of old-young the same = a great thing for the planet, and those 4bn would probably be happier and the whole world would be better.
But that isn't happening. You are going to go from a healthy split of 8bn, to a 4bn that is heavily skewed towards the elderly, and cannot sustain itself.
2 - Wouldn't it be better if we all died out. Better for who? For the planet? A ball of molten iron and rock floating aimlessly through space? So what. This idea that its better if we die out is just 100% virtue signaling crap. If you actually believed this, you would do certain things that would be highly immoral and unreasonable.
3 - it is ok because we have immigration. No. Those countries also have falling birth rates. Most of the world is now bellow replacement, and everywhere is trending down. Once those populations fall, where will your migrants come from?
4 - it is natural for animal populations to rise and fall... yes, usually at great suffering and cost. Most falls in animal populations come from starvation, famine, predators, conflict, natural disasters etc. Just because it is natural, doesn't mean that it is desirable to live through the effects of rapid population decline.
5 - But the climate??? Every new child born is a new brain that can potentially help the scientific world invent new technologies and solve the climate crisis. Without the youth we have no chance of getting out of this mess. I would argue that a gradual decline away from the energy and inventive nature of young populations, and towards a smaller, but gradually aging and tired population, is one of the worst outcomes for the environment.
For me, population decline is like climate change was 20 years ago.
Early 2000s, if you said that you were worried about the climate, no one really cared. Global warming was a widely understood idea, but no one actually cared and you were often laughed away for proposing any real change. Today it has penetrated the mainstream and we understand its a big threat.
Population decline will be the same. The numbers, logic, research on it is very very clear. But it hasn't really penetrated the mainstream yet, Give it 20-30 years, when populations are falling in some countries, bigger problems are happening in Korea/Japan, pensions are under a lot more pressure, schools are closing at a faster rate, and migrant flows are being impacted by countries not wanting to loose their young workers - and then it will probably be something most people understand.