If economic activity is linear with co2 production, the crash would need to be the most extreme economic depression in history to have an impact eg 75% reduction in global GDP. A 75% reduction in food production would surely cause the largest global famine yet recorded.

This will never happen willingly. Whenever it does happen it won't be by choice and will be because civilization has run out of material to produce stuff or too much of the Earth has become inhospitable. At least in the extreme long term it is a self correcting problem.