I don't know if this has been theorised on here before, but logically, if the upside down is all a hive mind then anything or anyone connected to it should die when the main source of its power is destroyed or defeated. Ergo, if Vecna and the mindflayer are defeated in season 5, logically El, Will and Max will die because they are also directed connected to them and the upside down.

Also, another theory that's been brewing for a while now is that all of this has some connection to Hopper's daughter Sarah and her death. I don't know how exactly, but I've noticed subtle little hints of it throughout the show, but to be honest I can't be bothered to go back and find them lol. Maybe I'm reading too much into it, I don't know. I could be wrong, but usually the Duffers don't add details without there being a reason behind it.

What do you guys think about these theories? Your thoughts?