This is an unfortunate delusion that is widespread, and exploited by governments and industries that seek to ravage what is left of our environment for profit.
If you believe and understand that the earth is a finite place, with finite resources (as all intelligent, rational people do), then you believe and understand that this finite place, with finite resources, can only support a finite population. Of course we can debate about what exactly the "sustainable" population is and we can agree that the "sustainable" population depends on how resources are managed, used and maintained, but there can be no disagreement that this number exists, and that if there are too many people our finite resources cannot sustainably support them no manner how efficiently they are distributed.
Unfortunately far too many people don't believe this, and don't understand that the earth is a finite place with finite resources. They insist that the earth can support an infinite number of people if only we manage our finite resources properly and impose a strict enough dietary and behavioral regiment on the teeming billions stuffed onto the planet.
A simple example of this is that water isn't used up, it gets dirty. It can be made clean again but that requires energy. This can be done by humans (water filtration) or by nature (evaporation and rain). We don't manage these cycles very well and they sometimes stretch out over too many of our lifetimes to manage (plastics, some nuclear waste) so it becomes easier to talk about resource 'use'.
The equation is pretty simple `humans × resources/human`. We can talk about reducing number of humans or reducing the resources needed per human. If we manage the cycles well, humans could inject more resources into the system instead of taking away from it. Of course this would still be limited by available energy. In that case, increasing the number of humans within energy capacity could benefit ecosystems.
We already have a lot of available energy but there is orders of magnitude more available as our technology improves — fusion, thorium fission, solar, wind, tides.
In the US we build shelters either out of carbon-intensive materials or use significant carbon fuels for upkeep... or both, usually.
It's not that the technology isn't there, we just choose not to. Hell, some 30% of people love that reality star turned president. Sadly we're getting what we deserve.
Changing our farming methods to increase humus and topsoil quality should bring back vitamins to our food. Not only this but it should also capture a great amount of CO₂. I'm not sure how reliable my memory or the original calculation are but I remember reading in "The Scientist as Rebel" that if we increase our topsoil by 2 inches on currently farmed land, that should capture most of the carbon we've emitted during industrialisation.
Of course, as you mention, if we keep mindlessly following the status quo, we'll keep getting what we deserve.
Good. Now read what I wrote again.
Side note on tone: The posturing ("delusion", "intelligent", "rational") makes you look bad. And it's like a thousand times worse when you're trying to pick a fight with someone by argumentatively agreeing with what they said to attack a straw man. If nobody ever told you that before, I'm sorry that you never got the benefit of that advice, and I hope it helps now that you know.