Here's one of the best argument I have heard on the subject:
The world, over time, will improve, will grow and will be a better and more efficient place to live than it was before. If that assumption turned out to be wrong, we have a lot more to worry about than the value of our stock portfolio. ...... ~ Bryan Rich, Forbes.com
I agree with this thesis. Just look back at last 30-40 years and think about all the time - people have said it's over and even then mankind has come back with innovative ideas and solutions. You can be totally pessimistic with Govt and leaders but if history has anything to go by: You can never be short on Human spirit. It will fight and rise again to NEW HIGHS.
This is my personal opinion and you are free to disagree.