What exactly do you mean mediocrity?
Do the people in the "Super Bowl only" crowd realize that Aaron Rogers, a first ballot HOF who was developed perfectly by his organization and is one of the most talented players to ever play the QB position has played in one Super Bowl in his entire career. The same amount that Rex Grossman and Brad Johnson have.
Going 10-6 isnt mediocrity - getting to the playoffs is what is important, and then the TEAM and QB have to play well to win a championship. Joe Flacco is FAR from elite and certainly not worth the money he is being paid, but he played 4 games of mistake free football, his defense played great and they won a championship.
No one can see the future, but if Matt Ryan can go 14-2 and make a SB, why exactly cant Kirk Cousins?