When a main character ends up departing and it wasn't pre-planned, shows scramble to put a Band-Aid on it. X-Files hadn't been interesting for two years by the time David Duchovny left, and filling his absence with Robert Patrick's steely gaze did nothing to make Season 8 of The X-Files seem like a bad spinoff of Season 2 of The X-Files.
20th Century Fox Television
When the T-1000 can't save you, you're truly doomed.
Steve Carell leaving The Office gave us a performance by Will Ferrell that may as well have been replaced by Will Ferrell screaming, "Like me! I am THE FUNNY MAN!" at the camera for four episodes. Even The Andy Griffith Show tried to replace Don Knotts with Jack Burns. If you ever look at a grandparent and they have weird, silent horror painted across their face, they're not remembering the war or any past trauma. They're remembering Jack Burns on The Andy Griffith Show.
Even worse is when a main character refuses to give any kind of shit for the remainder of the show's run. Jason Segel's performance in the last season of How I Met Your Mother is the acting equivalent of a second-grader counting down the minutes until the bell rings. The closest thing that I can compare it to is Harrison Ford in Return Of The Jedi. If Ford had been on set for just an hour longer, every take would be him walking over to George Lucas and spitting on him.
"I'm sick of this franchise."
In some cases, when a character leaves unexpectedly, the show can actually benefit from it. When Paul Schneider left Parks And Recreation at the end of the second season, the show became even better, mainly because the writers didn't have to wonder, "Should Paul give a slightly amused glance or a droll glance? Guess we'll leave it up to chance," at the end of every joke. Raymond Cruz, who plays Tuco in Breaking Bad, asked to leave the show way before the writers were through with him. And while it would've been entertaining to end every Season 2 episode of Breaking Bad with Tuco beating another one of his own henchmen to death, we may have never gotten Giancarlo Esposito's Gus Fring, who to this day is the greatest Lex Luthor in media history.
Sony Pictures Television