The problem is millions die, government's are no longer able to govern and popularist war lords gain power in the chaos which results in huge conflicts that cause far more ecological damage without any measures or efforts to remedy them - we still get clomate change but probably sooner and worse.

Also let me ask the people here with children who among you would let your child freeze to death and who would chop down a tree to burn? The ecological damage done by a civilization collapse would be intense, we're too close to the edge to risk that - maybe if it'd fallen at the start of the Industrial revolution but how long would it have been until that technology comes back and we're right where we are?

We need to change society and evolve new technology, the later is actually doing really well with many giant leaps for climate friendly technologies and infrastructure but society is proving to be very resistant, people aren't going to create a new greener world if they get angry at the very idea of being told to reduce, reuse, recycle.