I remember binge watching _House MD_ in college, really liked the beginning, and felt disappointed as the characters grew increasingly comical and less nuanced in later seasons. It's still the same show, similar vibe, similar character if we view them as symbols of personality adjectives (the sarcastic are still sarcastic, the sympathetic still sympathetic), but somehow among those similarities, the subtleties died.
At some point of the show, every angle of romance, mystery, character growth has been investigated, and none of them can come to a resolution, because, well, the show has to go on. How do you raise the stake---which is usually how you hook the audience---when they know nothing the character does will ever come to a conclusion, anything they become can be reverted in the _next season_? In fact, even the changes that have already happened in earlier seasons can be rolled back. I think this is why, in later seasons, a common complaint is that the writers are undoing everything good about the show in the beginning.
Some shows sort of avert this by replacing the main character (e.g. _Doctor Who_) and effectively telling a different story in the same universe. Does it work? Maybe. But still, I've, generally speaking, grown quite reluctant to watch TV shows that are longer than 5 seasons since then.