It's short-term thinking. You may have experienced this when procrastinating. Sure, we could roll back our lifestyle, avoid buying out-of-season/imported stuff, use public transport, fly less, build renewable power stations and avoid buying unneccesary stuff, but we quite like living the way we do. So we decide "Hey, let's wait until 2020 to sort this problem out!". 2020 comes, and instead of having (pun intended) 20:20 hindsight and realising that the situation is worse and acting upon it, we think "Hey, I think we can wait a few more years!". Typical.
I don't think we'll become extinct due to this (the only human-caused scenario I could think of is a massive nuclear war leaving tons of fallout and no health services), but if we mess up, countries will probably be fighting each other for resources like the last of the oil and of course water. There will be far more people than the Earth can sustain, and naturally our population will decline. Possibly to a few billion, possibly to a few hundred million. And our lifestyle will probably be set back several hundred years.