> Optimism is irrational

That is an argument of the pessimists and enemies of the good.

Pessimism is clearly irrational: Look at the world we live in; look what humanity has achieved since the Enlightenment, and in the last century - freedom, peace, and prosperity have swept the world. Diseases are wiped out, we visit the moon and (robotically) other planets, the Internet, etc. etc. etc.

To be pessimistic about our ability to build a better world is bizarre.

Pessimism and optimism are philosophical perspectives (dispositions) and do not necessarily have anything do with doing good or doing bad. Why do you think optimism only precipitates good things? Surely you can imagine a situation (or many) where thinking more positively about a situation than the data warrants leads to bad outcomes?