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.
You see that a lot for sure. One show that stands out in my mind of continually moving the plot forward is Breaking Bad. The ‘villain’ changes throughout the show, Walt has an evolving relationship with Skylar and Walt Jr, and motives change as well for Walt and Jesse. Completely different show from beginning to end.
SPOILER: it’s a well known fact that season 1 was shortened due to the last writers strike and had that not happened Jesse would have been killed off at the end of the season. This show very well could have suffered the same fate OP prescribes to most shows since Jesse is so pivotal to the shows overall success. So maybe we should look for shows who’s first season is currently being cut short by this writer’s strike and that will be the next great show.