We need both, to reduce the total number of people AND to reduce the per capita consumption.

I have seen more than enough people that claim to have no children for the sake of the Environment, only to go and buy pets and treat those like children; retarded children that will never learn to speak, walk in 2 feet, or become productive members of society, demanding constant attention for their natural lifespan, and which will age rapidly and die in their teens, - and have valuable medical resources expended in heroic efforts to futily prolongue their agony for weeks/months. You could claim that the pet dies early and does not produce offspring, but then the pet owner will likely mourn for a few weeks and then go to the pet store and start all over again.

Well, I mean, really, the only ecologically sound thing to do is kill yourself.

We don't need to do anything, the economy is already decreasing population. The dual income family has destroyed big families. It's very difficult financially to have one child and two is about the max families can handle. And dual income is necessary to survive now. Economy is getting harder and harder for everyone. The optimal strategy for families now is to put massive resources into 1 or 2 kids and hope they do ok in this coming automated economy world. Social security is on the path to collapse. It's not a giant savings account like many think. It's designed so current tax income pays for current retired generation. When the millenials retire, the system will collapse due to lower young population. But it's not just social security, the whole economy is designed like this. Population growth was a base assumption that is no longer true.

It's only in the well developed western world.

Third world countries have the 'capacity' to almost double world population (in case of medium growth scenario, hopefully population size will plateau at around 13 billion, I do not think that optimistic rapid decline is possible without rational action of global intervention to reduce population growth [0]), which IMHO sounds catastrophic. Because for next 50-100 years global population will be increasing rapidly and hopefully will stabilise after that, without any prospects of reduction for probably double of that time frame. Even if the population will start to decrease I think it may take at least few ages to reduce population size to what is today.

[0] https://en.wikipedia.org/wiki/File:World-Population-1800-210... , https://en.wikipedia.org/wiki/World_population_estimates