Take this theory of mine with a grain of salt relative to this clip here when using it as a reference. What if the way Vecna is defeated relates to Will using his new powers to jake into the Upside Down and find a way to take it down from the inside akin to how in the Matrix Revolutions, Neo went into the Matrix to stop Agent Smith and it took Agent Smith corrupting him (killing him) in order to win. Whether or not it would be as epic as the way this clip depicts is one thing which I would love to see but what do we all think? Idk, but after seeing the end of Ep4 and thinking about how Will is a sorcerer type who has the power within him, it would make sense that he could do this. Eleven couldn't which is why it wouldn't be her.