So I started writing this as a response to Sarah J in a thread in the Simpsons Hobbit couch gag story, and it sort of took on a life of its own. What do you guys think?
I have this theory about long-running TV shows. They always go through three distinct phases:
1. Story-driven episodes. In the beginning, we really don't know a whole lot about the characters, so the writers have to focus on interesting stories or plot developments. In the old days of television, this was the model for every show, regardless of genre. You could watch them in any order and never feel lost; you could start watching at any point and understand exactly what was going on. Today, this approach usually defines the first season or so, though usually it just means characters or plot devices that are never seen again. Remember how many early episodes of Friends involved Marcel the Monkey? Or that guy who was supposed to be Leslie's love interest in the first couple of seasons of Parks and Recreation?
2. Character-driven episodes. If the show is popular and renewed for more than one season, the viewers, writers and actors will get more emotionally invested in their characters and start to delve into their backgrounds and relationships. The first season of X-Files was mostly about Mulder and Scully looking for monsters in the wilderness, but by the second year we learned a lot more about their personal lives, histories, families, acquaintances, etc. Generally speaking, the more popular a show is, the more character-oriented it becomes, which is true even of shows we think of as formula-driven, like Star Trek: The Next Generation. Moonlighting started out as a comedy about mismatched investigators, but by the second season it was always about David and Maddy's will-they-or-won't-they? relationship, often to the exclusion of everything else. This development can lead inevitably to...
3. Show-driven episodes. If a series lasts long enough, say, over five or six years, it starts to be about the show itself. Episodes refer back directly to earlier episodes or plotlines, either out of nostalgia for the days when the show was fresh and new, or in an attempt to reinforce continuity (which is kinda the same thing). Often it stops being "about" anything at all beyond the cast. Once Bruce Willis became a big star, Moonlighting dissolved into solipsism. M*A*S*Hstopped being about the horrors of war and more about the wacky antics of the 4077. All in the Family stopped being about the generation gap and became all about how Archie Bunker was a lovable curmudgeon. Some shows stick to formula but end up becoming self-parody, like Law & Order: SVU. Other shows start resorting to weird magical realist stuff or improbable deus ex machina plot developments to keep the narrative going, like Gary's ghost on thirtysomething, or that whole season of Dallas that turned out to be a dream, or basically anything in the second half of Lost. Basically this is where the snake starts to eat its tail. Doctor Who has reached this point often in its 50 year (well, more like 35) history, but it has the built-in capacity to reboot itself at any point in time, while maintaining the basic premise.
I think The Simpsons has been caught in this phase for a long, long time. Everyone expected the show to wind down by the late '90s — David Cohen has said that when he was brought on as a writer in the seventh season, he thought it would be over in a year or two — but instead it just kept on going. Unfortunately, to keep on for 25 years it just keeps recycling all the stuff that made it good for the first eight or nine seasons, often with no rhyme or reason beyond simple nostalgia for what used to work. Most of the current writers were in grade school when the show was in its Golden Age. They remember what they loved about the show, and you can see them trying to ape that in the new episodes. But what made the show good was the way the original writers were using it to reference then-popular culture, or as a way of talking about their childhood frustrations or unhappiness. This is pure meta-commentary. It isn't really about the "real world" at all anymore.
(Keep in mind about half of these references are pretty old, since I haven't watched a whole lot of network TV since the '90s, and some of it is a working out of a paper I did in college over 15 years ago. So if you can think of newer or more relevant examples that support —or refute— my theory, please share.)