Maybe you should get out of the Ruhrgebiet more often. Pretty much all German cities that implemented Fußgängerzonen (pedestrian only malls) I can think about have thrived, big and small ones. I don't know what time frame you are talking about, but most cities in the Ruhrgebiet have been on a downward spiral since the 80s, so we might be looking more at a case of correlation not causation.