I think the problem with your idea is that there's huge lag in the system. Even if all human CO2 production ceased today, there would still be huge changes for decades to come. A global catastrophe due to climate change would eventually put a stop to further climate change, but by then it'll be a little too late.
It's a bit like arguing that you can't die from jumping off a bridge, because then you wouldn't be alive to make the jump. It misses the important temporal relationships.
Right, that's a big part of the lag. If climate change causes civilization to collapse, it'll still take decades or centuries for the built-up CO2 to drop back to normal levels. Plus the fact that emissions won't stop right away just because a bunch of people start starving.
It's a bit like arguing that you can't die from jumping off a bridge, because then you wouldn't be alive to make the jump. It misses the important temporal relationships.