Cartoonists have typically engaged in what I like to call "long form work." Not long form as in Tolstoy-800-page work, but more like George Herriman one strip a day including sundays for 31 years work (11000 pages, for those who like numbers). When a strip runs for that long (and I am really referring to newspaper comics that are the work of one author, not strips like Garfield that employ teams of writers and illustrators or strips like Blondie that continue long after the creator has died) the reader develops a deep, abiding relationship with the characters and the individual world of that comic. The best of comic books enjoy that relationship too... what reader of Tintin doesn't pine for a world of such clean simplicity and elegance? Or the Marvel universe of wisecracks and effortless muscularity?
The cartoonist, as creator, has a duty to consistently serve the reader. Just because the world is the creation of one individual does not necessarily mean that it isn't shared by others. When a cartoonist throws in the towel and quits drawing their comic, they essentially shut the door on that world for everyone. Peanuts, Calvin & Hobbes, the Far Side, Bloom County and, most recently, Achewood, have all gone away because the creators decided for one reason or another that enough was enough (although Peanuts ended because of Sparky Schulz's death).
Part of me understands this... I have labored on hundreds of strips and have yet to break through in any way... but part of me is really incredulous. It's hard work, sure, and there is a possibility of failure and mediocrity in every panel, but the risks of continuing on outweigh the lameness of quitting. Every case is different: Bill Watterson battled for years with his syndicate and was burned out, Gary Larson thought he was going crazy, Breathed phased himself out gradually and Onstad said he didn't want to parody himself. Quitting, though, is quitting, and the reader is left holding the bag.