Yes, the worst case is pretty bad, both for us and life on earth. We're currently in the middle of a large extinction event, so the biosphere is probably pretty fragile even without the ongoing global warming.

If it "helps", the last time CO2 levels were this high was in the middle of the Permian. 3-4C higher than today, 10C warmer at the poles, and sea levels 5+ metres higher: https://scripps.ucsd.edu/programs/keelingcurve/2013/12/03/wh...