Well the problem with defeatism is that it's a self-fullfilling prophecy. If you believe everything is lost you will not try to enact change.

Now I acknowledged already that it might well be too late, but I also maintain that there isn't enough certainty in that prediction to base your actions around it.

So the reason to not be defeatist is twofold. There might be a chance to reverse or at least lessen the impact of climate change, and by being defeatist you are robbing yourself and future generations off that chance.