One one hand, in the 7 seasons Bogut has been in the NBA, the Bucs have been to the playoffs (1st round) once. On the other hand, in the 7 seasons Ellis has been in the NBA, the Ws have been to the playoffs (2nd round) once, too.
One player doesn't make or break a season. The whole team determines how will the season will go.
To say the Ws have underachieved just because of Ellis is just as ridiculous as saying Bogut will take them to the playoffs. They can potentially be good next season, but who knows. One injury to Curry, Lee, or Bogut will ruin all these optimistic plans.