I disagree about 7th Heaven. I definitely feel like the show went into deep decline immediately after season six. Starting with season 7 the characters, dialouge, storylines and the absence of Matt and Mary swiftly hurt the show. Another crucial point was the erasing of Robbie who became an honorary Camden over the past few seasons. No explanation of what happened to him as well as just erasing a really cool character. Other than that I totally agree with your examples because there is a distinction. I'll give you one example of my own with a tv drama: The Blacklist jumped the shark big time when Megan Boone left. The whole show was the Red\Liz dynamic and without her the show immediately lost its core aspect as well as its biggest mystery drive.