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.
Depending on the kind of show it is contextual, but here’s some.
If it is a tight self contained story that ends…and then more things happen. Stranger Things for example pretty much perfectly ended in season 1. There was a tiny dangling mystery regarding Eleven’s fate. Such things do not need to be a sequel hook, they can simply exist to tantalize and never be expanded on. This is like if Inception 2 was made and it answered the questions about Cobb’s spinning totem; it would utterly miss the point that the story was over and the ending was intentionally ambiguous.
If the actors or voice actors are simply getting too old for the part. Personally I have a good ear for animation’s voice acting. It drives me absolutely crazy when I hear noticeably aged actors reprising role or continuing them as if nothing has changed. Obviously some performers can last longer than others but for example modern Simpsons is unwatchable to me entirely on the basis of the voices. Even if somehow the writing turned around I simply can’t get past the voices. Similarly I could barely sit through The Incredibles 2, which supposedly picks up right as the first movie ends but all the voices are aged 14 years and I can hear it.
I’m kind of ambiguous about the first point. I think you can expand on a tightly-written, concluded story… but not repeatedly. Furthermore, it requires you to - to some degree - shift the focus of the following stories. Continuing the meta-story is all and just fine, but the immediate story can’t be about the same theme/issue/encounter indefinitely.