I think Arnold Constable closed the same year that the Bergen Mall was converted from open-air to an enclosed "true mall". Was it 1971 ? I was a kid at the time, but all the talk around town was that those two events marked a big shift in where people would shop. How true. I think that was also the same year that Temple Beth-El moved from State Street to Summit Ave, which was another bellweather event that had a big long-term impact.
1971 was when Main Street really started to decline as a commercial center. I think the enclosure of the Bergen Mall (followed by the opening of Paramus Park in 1974) hurt Main Street as much or more than the initial construction of the Garden State Plaza and the Bergen Mall (as open-air malls) in the mid-1950's.