The greats were great. That's why they're called the greats.
Dan said it well that you have to put yourself in their shoes, and be in the right context before you look at work and judge.
I sometimes complain that it's difficult to carry a camera bag around, a tripod, and be fully dressed for winter shooting when it's three feet of snow outside and temperatures in the 'It's bloody cold out here Mr. Bigglesworth' realm. Compare that to a situation of taking pictures in a war zone, with no choice but to go on. Dirty, tired, thirsty, possibly suffering emotionally difficult environments, possibly not knowing where to be safe, or where your next meal is going to come from, you still have to worry about making good photographs that serve the documentation of war. You would be pretty great just to survive. And lucky.
As far as somebody like Cartier-Bresson, well they only show us the best work. There are thousands upon thousands of terrible ones too, but they had the skill to recognize the really good ones. Those photographs were made in time where they had access to certain materials, and they worked with them to the best of their ability. You must, must, must look at the content, how it describes the subject matter in that time, and what they tell us.
I was just at the Harrison PHotography Gallery at the Minneapolis Institute of Arts to view some of the donations that Martin Weinstein had given the museum over the years, and among the prints were a lot of Ruzicka photographs. Many of them had big blotchy areas, full of artifacts added by the process, but you look beyond and see these intense beautiful highlights, and depictions of objects in a time before World War 2, for the most part. You have to look beyond the little technical flaws, which can also be brought about by improper process and generally the hand of time.
Pay respect to those who helped form the genre of photography as art before us, by realizing what they contributed, instead of finding flaws.