‘Tarnished for ever’: why don’t great TV shows ever end well?

As an avid fan, I enjoyed how Breaking Bad wrapped things up. Checking right now, it has a 9.9/10 rating on IMDB as well. Dexter, one of the examples in this article, was falling apart before the final episode. The Simpsons, unfortunately, may end up having more bad episodes than good. Thanks to a user on Reddit, you can see the badness creeping in visually https://i.redd.it/edv7vzciu8r41.png

Info: 'Line of Duty' screwed up the ending and pulled a Game of Thrones, but apparently not a St. Elsewhere.

Six Feet Under and ER had good endings as counter examples.