I think you can put together a list of counter examples pretty easily. Blind optimism is perfectly fine for making progress, in the cases where the action taken turns out to work.

Plenty of people started businesses on what may be termed blind optimism and succeeded.

I think it even helps a little to underestimate the obstacles in innovation - otherwise one might not attempt it in the first place. But just a little, because diverging too far from reality brings its own problems.

Optimism is essential to progress.

But we can all build a better future together. :)

You can build a better future with or without optimism. You cannot build a better world if the default position being communicated and adopted is cynical and defeatist.

Let's agree to be cynical about the motives and results of institutions that are by and large out of control, and to be optimistic about the outcome of our own individual actions.