re: The problem with nearly every TV Series ever made

Depends on what you're going for, really.

Comedies, and shows you might call comedies but are really more just reflections and observations on daily life with a sense of humor, can go on a long time before getting stale. It just depends on the premise the show is built on.

Speaking of premise- you take Magnum PI (original), and that could go on as long as the actors and writers wanted to. Doesn't matter that it was mostly episodic in nature, because it was the ideal male fantasy (Thomas Magnum, the dashing ex SEAL, lives rent-free in a Hawaiian estate, driving a Ferrari, entertaining countless young hotties, hanging with his buds, jousting verbally with Higgins etc). Yeah, at some point he had to grow up.

For something with a more specific overall story, you do probably need to plot out the course of the show, and hit markers every so often to keep it moving, and when you hit the end, shut it down.