I believe that Miller is pointing out the flaws inherent in the system, rather than in the ordinary man who is crushed by it. Willy tries to have what he is promised- a happy marriage with loving sons; a job which rewards him for his hard work and the modern conveniences of devices which are labour saving and stress-free. Tragically for Willy, his fatal flaw is a belief that it is possible to even begin to succeed with the odds stacked so highly against him.
I don't think that Miller would consider Willy a total failure. I certainly think he would find the dream that Willy pursues as one where failure is present. The dream that Willy pursues, one in which success is defined in strictly economic terms, is one where there is a greater chance of failure present. Part of this is because a dream where a financial end is determinant of success means that there can always be more money made, meaning that it is infinitely regressive. Willy does not recognize this, in that he believes that money can determine his happiness. The delusion that is a part of Willy's character can be rooted here. When Miller argues that Willy's failure is "in the situation in which he stood and to which he was reacting, and which was reacting against him," I think it's an indication that Willy was not as much of a failure, but rather that the dream and the conditions governing this pursuit both predisposed him to failure.