With the exception of a few pieces, total roster turnover is necessary. There are too many holes right now, and we don't have the resources available to compete at a high level right now by spending. Prince Fielder is not going to make you a contender five years from now, he's a right now sort of signing. You only make that kind of signing if you're close enough where you think it puts you over the top, not if you think it will put you into contention.
Roster turnover does not necessarily equal tanking one or more seasons. We could have made every move we made this offseason and still have justified adding one or more of Pujols/Cespedes/Darvish. You don't just add major free agents when you're on the brink of being great, you add major free agents to make your team better. Right now, short of splurging unwisely on FA pitchers next year, I don't really see any way we compete next year. Had we signed Pujols (or Darvish or Cespedes), it'd be a much more realistic thought to compete next year.