Yes, now we begin to agree.
It's true that we could live sustainably, provided we stop using cars and planes for everything, we live in smaller flats, we eat local and seasonal food, and stop buying so much crap.
Even though we could, the real question is if we ever will do so before it's too late.
The heavy smoker could also stop smoking any day in theory. But in practice?
I hope you're right for the demographic curve. I'm not so optimistic. WW2 didn't change much the demographics curve for example. You really need a shitload of famine/war/drought to kill a few billion people.