Humans seem to have a way of surviving, you're right; and you're right about the population, if 75% of it disappeared overnight tonight the planet would be in a better place the next morning. Maybe 90% would be a better drop.
I commented on another comment on this post about what humans feel comfortable to do to each other, no wonder the plants, animals and planet don't stand a chance. Also, I'm a history guy man, war history is something I spend a lot of time on, among other things of curse. So, I get what you say about WW2 docos.
On sustainability, I don't know man, I'm just not that smart and certainly am not a scientist, I just don't see how our voracious overuse of things, the way we want more and more, could be sustainable in a world that's teetering on the brink as it is. Massive change is needed, I just don't see it being done. A thing here or there sure, but massive change (in better directions), nope. I hope you're right though, and it can be done.
We have the potential to be an amazing species, we just. Goose not to. Greed and hubris are two factors of why.
Time will tell, the next fifty years.