Using Dean as vessel to the primary villain of the season would have required Supernatural to reinvent in a good way.
Let’s face it: Supernatural is long in the tooth by TV standards now that it’s in fourteenth season.
While running for such a long time has given Supernatural a rich history and developed the relationships between the central characters in some compelling ways, the show does need to find ways to reinvent itself as time passes, and there are only so many times the Winchesters can stop the apocalypse.