We kept patching things and adding minor improvements for 2 years after release. An expansion adding some mid-game content like the hub above Maadoran wouldn't have sold much (10-15k copies tops).
I disagree about the sales, since people who bought Dungeon Rats are the same people interested in AoD. They would have bought any additional content.
I have more than 200 hours in the game and when I completed it the fourth time I put my hand on my forehead and thought: "Why the hell did they make Dungeon Rats? Who was asking for it? This needs a good expansion."
That's just my personal experience.
Not sure what you mean. Like any other city Ganezzar has over 15 faction quests ('over' meaning quest forks), which is a big chunk of work, plus the side quests I mentioned earlier. That "one" quest you mentioned consists of 5 quests, more if Miltiades get involved.
Maadoran has 6 districts: The main area where you get the arena quests, the protection quest, a massive fight in the desert, one or two encounters involving a trader; the slums, which have 2 big fights, the package quest and the awesome well adventures; The Abyss; the trading district which has the monastery quest, the gem quest and a crazy loremaster (which technically isn't a part of any quest, but still important); the noble houses which have only one quest that involves the armor. All of this are side activities.
You will also get access to: Library of Saross, Zamedi, Inferiae, The Arch, Monastery (also, possibly Dead River and Hellgate).
Okay, now we're arriving in Ganezzar and... well... there is only one quest line of the size of the protection quest line from Maadoran; a continuation of the armor quest with a big fight in the end and a small version of the arena. That's it.
You will get access to the Facility, Dead River and Hellgate.
That sums it up I think. Also, after the siege Ganezzar looks like nothing happened.