When you deal with any long running series, especially sci-fi and fantasy, be it books, movies, tv shows ect, you will run into the trope I call “Get out of ___ Free” or GOOF for short. The ____ basically can stand for anything that can end a series by taking one or more pivotal protagonists out. This usually means death, but it was mean a whole host of other things, like disfigurement, mind control, metamorphosis ect. Having your heroes permanently stay as monsters or dead is anti-climatic as the villain wins, story over. That’s where GOOF comes in, better known as Dues Ex Machina, something that instantly brings the heroes back to normal so they defeat the villain and/or the series can continue. There’s nothing with GOOF, I even use it, everyone does, there’s nothing wrong it. Your villains won’t be effective is they can’t actually affect your heroes. And unless you’re planning on ending the series, those characters have to have some way of coming back.
The problem comes when its used excessively. TV shows are the main offenders because they have to fill up to 22 episodes a season. By the time Xena: Warrior Princess went off the air in 2001, both Xena and Gabrielle had collectively died so many times death was meaningless. It was in fact so meaningless, Xena had to choose to stay dead in the series finale so she’d actually stay dead.
In Stargate SG-1 almost every week one of the team members would turn into some strange creature or come down with some mysterious illness and eithered be cured by the credits or the next episode if there was a cliffhanger.
Now not all shows were like this. Heroes and Lost are good examples where if characters died, they stayed dead. However, in some cases, it seems like they were killing of characters simply to create enough drama to fill a season. Lost season 2 is guilty of this in spades.
You can also find overuse is long running books series, RA Salvatore’s Drizzt books comes to mind and I think the Wheel of Time series as well from what I’ve gathered(but don’t quote since I didn’t make it past the first book).
In video games, GOOF overuse is standard operating procedure for most RPGs , unless we’re talking games with permadeath like Fire Emblem or Roguelikes. Otherwise, staying dead never happens as long as you have the inventory items or can make it back to town… until the plot states that they die permanently(Hi Final Fantasy VII).
As for my own work, each character when created, they get a “can die” checkbox because once you die in Terrall, bye bye. Most of those unchecked are pretty obvious, but there’s one side character in particular who also in unchecked, who that is I won’t say.
Now, why did I infer that my main character won’t stay dead? Because it should be obvious in properties where the franchise is built around specific characters, as K23 Detectives is built around a central set. If a story takes place in Terrall and doesn’t include them, then its a side-story. Also, if James Bond dies, James Bond is over. Superman was never going to stay dead. Captain America did die, but wait, someone else is taking up the mantle.
However, no one told Chris Carter to end the X-files once David Douchoveny left, as Mulder-less X-files sucked as does Grishom-less CSI. While there are cases where main-character switching improved a show, like Shannon Doherty leaving Charmed but that’s usually due to the actor playing the character, not the character in itself. In these cases, another actor will play the character if they are pivotal enough(James Bond, Dr. Who, Superman, Batman ect).
In the end, whether or not a character should exit a series or not depends on a few factors. One, can the main character leave and the series still be the series? This usually only works in a few areas; long-running procedural where the characters have little development beyond doing their jobs or in super hero comics where multiple people can assume the same heroic identity. This can also work in Soap Operas which take place over long periods or time, but they often suffer from GOOF overuse too.
Second, if the character is expendable, does the character replacing the original one either equal to or an improvement over the leaving one? Depending on the character, you might not even have to replace them. This is often telegraphed at the start of series where a “main character” seems a little less developed or out place, this is rampant in established franchises. Morph in 1990s X-men animated series is a good example.
Finally, is there a good reason why you even have to kill them off if you plan on continuing? Killing main characters is a risk few series can survive intact, which is why GOOF often exists in the first place; to have drama, to have a character die, but still keep them around.
In the end, this post can be boiled down to one thing: Kill your babies at your own risk.