Season 1s are great, setup, some payoff, a bit of lead into the overarching story. Then season 2 to X. The heroes win and then lose in the final episode, cliffhanger to next season. People get bored. Final season is announced and they wrap up the show.
I think the point of the question is that the producers get greedy midway. And stop the progress so they can go indefinitely. Then when the show is cancelled the finish the story arcs in the final season.