It also wouldn't surprise me that the same "stimulus" is lost. S1 left us confused about the Upside Down and the creatures contained therein. S2 and S3 were less mysterious about them and included less truly "strange" things. I think the "problem", albeit I enjoy it, is that Stranger Things is really a homage to classic science fiction and horror tropes. That's why it is full of 1980s nostalgia and chronic references. Once that "ooooh" is lost... it really just becomes another typical "horror" television show. You won't be scared or tensed as the monsters aren't new and your favorite characters have plot armor. I mean.... in S1 only a handful of people died. In S3 it looked like the entire town got slaughtered. You can see they're reaching to try and up the ante and it's not working. Hopefully they go back to focusing on the personal horrors of the characters --- not trying to make it a world ending scenario. It has just been done and happens with every show that starts with a dark premise --- gotta "save the world". Again. And again.