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.
Atlantis was, if I recall correctly, intended for a while to be the successor. The plan was for the SG-1 show to end with the Atlantis mission beginning, and then the Atlantis show to be the next stage of the Stargate franchise. What ended up happening was the Atlantis mission kicking off but then also TV people in charge wanted to keep the SG-1 show going so you had the shows airing at the same time. That is partially why SG-1 mildly turned into a zombie version of itself. Certainly not as bad as other shows, but I could still feel it.