My personal sign is when you start seeing awkward collaborations start cropping up. One time when I was thrifting, I picked up a graphic novel that had the Justice League, team with the Power Rangers of all things. I glimpsed into what the plot was about out of morbid curiosity and it was just a plain generic time and dimension thing.
Nothing ever connected between the teams at all. DC Comics, while fledgling at times with how they go about their series and movies, still have far more relevance than Power Rangers do. I think the Power Rangers are just grasping at straws to keep being relevant when people have largely moved on from them.
Absolutely an issue for American shows. So many are built around a three season arc just trying to get on the air. Then if they make it big, they panic and have no idea what the fuck to do with season 4.
So many shows fail in season 4.
Probably the worst example is Game of Thrones. They started the show with at least the last third of it unfinished, and once that TV money started rolling in, GRRM basically lost his motivation to write.