jabird, you're spot on with Airport City just being a large mall and its purpose is to create more revenue for the airport owner. The concept of an airport city, also called aerotropolis, is now a close to 20 year old. In 1992 Mall of America opened just outside the perimeter fences of Minneapolis-Saint Paul International Airport, and this was three years before the French anthropologist Marc Augé came out with his essay "Non-Places: Introduction to an Anthropology of Supermodernity". The expression "non-place" is difficult for us aviation enthusiasts to swallow when used about an airport, but if you want to read more about airport cities - here is a short and precise wikipedia page about the subject:
Airport City - Wikipedia, the free encyclopedia
I have been a regular traveler through DUS from the late 1990s (after the horrific fire on 11 April 1996), but it latest until 2005 for having enough time to visit the roof terrace. On that May day in 2005 my impression was that a large portion was English speaking, and a group of ten had flown in from Manchester that day. I have never given the impression that the plane spotters play an important passenger group between MAN and DUS, especially when you also take in to consideration how good spotters are to find cheap tickets.