Famously, Game of Thrones. (which I never watched).
Naruto, before it turned from a series about ninjas, into Dragonball Z.
Dragonball Z, after the Cell Saga, with the handoff to Gohan.
Star Trek Discovery and Picard. Should have never started.
Stargate SG1, after the final defeat of the Goa’uld, before they introduced the Ori.
The Mission Impossible franchise should have never had movies. From the beginning, it just was “Tom Cruise’s Midlife Crisis – Action Schlock Edition”.
The MCU should have arguably ended with “Endgame”.
Star Wars should have ended with Return of the Jedi. No prequels, no sequels.
Agents of Shields should have ended after the defeat of Hydra. Definitely before the time travel to the future of space.
The Terminator franchise should have ended after T3. Because I like the ending of “the apocalypse is inevitable”.
Sword Art Online should have ended after the Aincrad arc.
Cobra Kai, after Season 2. In season 3, the incessant teenage love triangle melodrama was really pissing me off.