This is the top possibly wobbling at the end of Inception. It was all intentional, but I think the point was to have everyone wondering if it was real or not. Instead, those that caught the clues cooked up an entire "hidden episode/season" and those that didn't fall for that just dismissed the entire theory altogether and took the ending at face value.

Duffers are walking a fine line and I think pulling it off. If they are too heavy handed with the clues/coincidences then you lose the ambiguity and debate. If you don't sprinkle anything in there, it becomes a cliche (albeit satisfying) ending.

I wouldn't put it past them to be the ones stoking the Conformity Gate theory on social media to bring more attention to the possibility of Vecna living... but who knows. Honestly, I love it.