As the rare DF player who focuses on adventure and legends mode more than making forts, DF is not *quite* at the point were I can play it for long periods of time. So other games still keep their appeal. I think this next release might push it over though. Living world = play the same world for RL weeks, then gen a new one and get a fresh experience.
My take other game series is, they're constantly trying to keep up with modern graphics. This requires them to essentially rebuild everything from scratch each new game, and then go a little further on the game mechanics. I guess that makes sense for games that really use their graphics, like Mirror's Edge (not that it even got a sequel) or Bioshock, but for the average "real is brown" game better graphics is mostly just advertising. Game devs that constantly update the same game rather than putting out sequels can achieve a lot: other rougelikes are the best examples of this, but MP games like TF2 can get a lot of content with unusually good balance if they take their time.
That being said... Toady is really good at what he does. He's built the most complicated game ever on his own, without using 3rd party products the way other series do (well, there was some help on the graphics code, but the vast majority of the coding is just Toady). I know Toady has sometimes portrayed himself as a sloppy coder... but I'm not sure I believe him. He's certainly succeeded where others have failed.