I've heard the opinion expressed that optimism is incompatible with objectivity. In other words, anyone who questions the current "sky is falling" pessimism is clearly blinded by subjectivity and can therefore be ignored safely.
I would say that optimism and pessimism are only problems when they come first, when they determine what facts one will accept or reject. If the facts demonstrate a trend of improvement (as I have argued is the case for humanity generally), then to be optimistic is not some kind of flaw in one's reasoning; it is to be happy as a member of the human race that one's species has a chance to endure and thrive.
Objectivity is a good thing in analyzing something important. But it's neither necessary nor appropriate when the results are positive -- human beings are allowed to be happy when they get good news. On that basis, I'll willingly accept being called "optimistic" regarding the results of analyzing human history.