Klein's work was revolutionary because it was casual and broke many orthodox rules of photography at the time. Today Klein's style of photography is the default for everyone with a smartphone or digi P&S.
Throughout the changes in photography history, the classic, looking through the viewfinder and critical work, from Lewis Hine to Gene Smith to Salgado has remained the backbone of serious photography. Salgado's Genesis for example is very orthodox and yet its the most celebrated post-digital photography work to date.
Looking through the optical viewfinder is what makes photography different from painting or any other means of expression. It is a unique way of looking at the world, everything is compressed and heightened, which allows for concentration, careful framing and therefore a personal touch to photos. That personal touch is missing in LCD or EVF shooting. Because one cannot get emotionally connected to a scene through a 3" LCD. EVF might help with framing; LCD shooting is a complete disconnect both stylistically and emotionally from the subject.