There are periodic protests by fans of some TV shows when they are canceled. The latest is about Netflix canceling the series One Day At A Time after three seasons because it said it attracted an insufficient audience. I had not heard about this show until the cancellation protest and people wrote in support of the show saying that it was well-written and funny. It is a reboot of a show of the same name that I watched when I was in graduate school and had the same general outlines of a single mother raising two children, and it even had the same theme song and the same name for the building supervisor. Part of the reason for the protest is that the reboot was one of the rare shows that featured a Hispanic cast, in this case a family of Cuban origin. An added bonus was Rita Moreno as the grandmother.
I am of the opinion that TV shows should have limited runs that are determined and announced in advance, because however good they start out, they tend to run out of steam and become repetitive. For example, the fourth season of that funny show Soap was awful and it was merciful when they abruptly stopped it, though the sudden end meant it ended the last season on a cliffhanger that never got resolved. I am worried that the The Good Place is going on for too long and that it may be time to pull the plug, even though I like it a lot. Arrested Development just ended its fifth season with what looks like finality and that is a good thing because it was running out of steam too. The show Galavant was excellent but was canceled after two seasons. Much as I liked the show, I was not sorry to see it end because it ended on a high note and I thought that it would become stale if it returned for more.
One cannot have a formula for how long a show should last. It depends on how many shows a season contains and how long each episode is. In the US, regular broadcast network TV seasons consist of about 22 episodes, which is a lot. The streaming services are not as locked in to a daily schedule and so are able to have series of varying length and this is having a beneficial spillover onto network TV. In an interview, I read that the creators of The Good Place argued for and got seasons that are only 13 episodes long (though even that is a lot) because they said that in a 22-episode series, the second half of each season is usually of lower energy as the writers and cast are just treading water until the end. I think it helps to have the story arc planned out in advance that has a definite end, like a mini-series. I am more likely to watch a TV series if it is of finite length. Ending a series also frees up room for new series to emerge that showcase new writers and actors and situations.
It is like comic strips in newspapers. There are strips that people are so attached to that they go on forever, even after their original creators have died. Newspapers either continue to recycle the old strips or new writers take on the same strips. This results in the same gags being recycled and leaves little room for new talent to emerge. In the local Plain Dealer there are certain strips such as Family Circus and Marmaduke that in my opinion have gone on far too long, One has to admire Bill Watterson who decided that ten years of his incredibly popular Calvin and Hobbes was enough and walked away from it. Bill Amend did the same thing with his funny strip Fox Trot.
When people talk fondly of past shows, that almost invariably refer to shows that had limited runs, like Twilight Zone or Fawlty Towers. I think that it is because those shows ended on high notes before the creativity ran out.