You are confusing and conflating a sudden loss of people with conservation efforts.
A population increase to 400+ million sure as hell isn't going to raise our standard of living.
What do you think increasing our resource demands by 25% will do?
We'll need 25% more cows. More energy. More wood. More rare earth metals.
How anyone thinks they can defend this is beyond my understanding.
Take a bloody look at it it means to increase our population as we have been doing:
https://www.theguardian.com/environment/2018/aug/23/they-are-taking-out-a-generation-of-tuna-overfishing-causes-crisis-in-philippines
https://www.nytimes.com/roomfordebate/2013/01/10/did-we-save-the-whales-19/overfishing-threatens-whales
https://www.nytimes.com/2017/04/30/world/asia/chinas-appetite-pushes-fisheries-to-the-brink.html
Humanity has wiped out 60% of animal populations since 1970, report finds
That gravy train isn't stopping. Our population is growing, so it is accelerating all these issues with consumption of resources.
Try doing some basic math on it. More people, more consumption. More restrictions.... less consumption PER PERSON. The first response will be to limit our standard of living. And while that is actually inevitable and cannot be avoided, the amount of pain we endure is based on our population. Fewer people, less pain, higher standard of living.
This isn't an 1800s farm where you need more labor to function. Automation is killing off labor in mass.
This isn't even the 1900s anymore. Again, automation with industry.
Nothing good can come from spreading ourselves thinner than we already have. Or by killing off the planet while doing it. Stop filling the balloon before it pops.