The introduction of Vecna as the main villain may herald the end of the incomprehensible abominations which made the first three seasons of the show so great.
The same secret recipe of horror movie references, talented actors, and 80s nostalgia that made it so popular in the first place will likely keep it popular. The scariest moments in Alien were when the crew of the spaceship Nostromo were still unaware of what they were dealing with, and the movie only showed a brief glimpse of it. The Mind Flayer is a monster that devours the brains of its enemies and enslaves intelligent beings to use as thralls. The monster in the first season was named after the powerful foe from the game, the prince of demons.
Vecna is a good choice in one way, but this stems from the same issue. The fear comes from the unknown and the lack of knowledge is a vital part of eldritch horror, popularized by authors like Brian Lumley and H.P. Lovecraft. The sequel, Aliens, couldn't be the same kind of space horror movie because the creature had become too well known. The new villain, Vecna, will attempt to be a replacement for the Mind Flayer from season 3.
The show's scariest parts are no longer scaring people the same way they used to. It can either introduce something new, or it can evolve into a different kind of story, leaning towards the action-horror which is a staple of many vintage horror movies. It is likely that it will never regain the kind of horror that made the first season so great. The show may be headed in a different direction with the arrival of Vecna. Vecna will pay homage to classic movie villains like Freddy Krueger from A Nightmare on Elm Street, with a much more human appearance than the monsters from previous seasons of Stranger Things.
The D&D games which the main characters are shown playing at the start of season 1 have always been referred to as monsters in Stranger Things. A demogorgon is no longer capable of provoking the same kind of fear. Vecna is a D&D monster that used to be a powerful human wizard. Once too much is revealed about the unknown, the horror loses its grip. The monsters from previous seasons can't be used in the new season. The true purpose behind the actions of the Mind Flayer was never revealed. The name Vecna is also known as the "Undying King". It is not saying that it will become a bad show, but it is saying that it will be different. A more humanoid monster is likely to have more understandable goals, making him less alien than previous villains.
A problem with Vecna is that it is best known for incomprehensible and demonic horror. They are horrified by the fact that there is so much unknown about them.