There is a difference between responsible criticism that aims for success and defeatism that refuses to acknowledge anything but failure. Hindsight alone is not wisdom, and second-guessing is not a strategy.
An artist must only judge of what he understands, his field is just as limited as that of any other specialist... That in his sphere there are no questions, but only answers, can only be maintained by those who have never written and have had no experience of thinking in images.
It's useful for activists just to be out there to keep you mindful and not get complacent, even if ultimately you think some of their criticism is misguided.