Let me start of by saying that I do not necessarily think optimism is better than pessimism or vica versa. However, your statement falls short of being effective.
You state that one is better than the other without giving it a context. For what purpose are you saying pessimism is better? Basing it stictly on health for example, pessimist are demonstrated to have higher general stress levels and resulting in poorer health - which would surely lead to the conclusion that in fact optimism is better.
Also, as proof you offer a worst case scenario - the failed hopes of the optimist, and compare it against the best case scenario - an unexpected success for the pessimist! Sure you can see how offering this loaded evidence is hardly convincing?