Series are a double edged sword. If you are an author or publisher who depends on sales of your books, series are the way to go. Readers who like one book in a series will generally buy more, so once you land a reader, it’s much easier to sell to him or her again than if you have a bunch of stand alone works.
If a book or movie makes money, people will ask for a sequel. And I get that. From a business standpoint, it makes perfect sense. It’s low risk to make a sequel to something that has already proven its popularity.
The problem is that some stories lend themselves to series better than others. It’s easy to thrown another murder at the main character if that character’s job is to investigate murders. It becomes forced when terrorists take over the airport every single time out hero takes a flight, or kidnap every new girlfriend, or a body falls out of the closet every time our protagonist checks into a hotel. This makes even less sense if the character is a florist or pastry chef as opposed to a spy or mercenary or someone who would attract enemies and moves in a world of violence.
Looking at you, Jessica Fletcher.
So, yes, we can easily have Spenser or Mike Hammer or Anita Blake solve new mysteries every year without unduly straining the suspension of disbelief. But what about the non-episodic story? What about the quest that never ends or progresses? This is a problem with a lot of fantasy authors. You start a series and really think it’s great, but by The Dental Hygienist of Shanara or volume 75 of The Wheel of Time, things have gone a bit stale.
A long series that tells a story can makes sense, but only if there is progress. The Harry Potter series takes six books, but they follow the characters through their time at school. They age, they mature, they change. At some point, that series has to end.
Remember, all good things must come to an end.
But sometimes bland and trite things don’t.