Eric Forman: bearable, hit-able guy.
And then he left to pursue a film career that has included the third-worst Spider-Man movie and first-worst film to ever be titled Valentine's Day. His void was filled by a character named Randy, and until we see the deleted eighth season episode "Randy Is Slowly Choked To Death By An Invisible Laughing Hand," I will never be able to forgive That '70s Show for what it did.
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Steve Carell leaving The Office gave us a performance by Will Ferrell that may as well have been replaced by Will Ferrell screaming, "Like me!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.