Complete rebuilds are actually not smart. The Cubs and Astros each won one world series (and the Astros had to cheat).
The Cardinals get a substantial portion of their revenue from fan attendance. If you look at the impact of tear downs on fan attendance, its devastating. It is hard to win back fan trust.
The Cardinals have a lot of money coming off the books after this year. They can invest that money in pitchers (Scherzer & Lynn, for example). Add a Yimi Garcia and the just released Kyle Barraclaugh for bullpen options, and now you have much more pitching depth. Plus, Oviedo and Woodford should be much better next year.
If Arenado opts out, you have Gorman waitiing in the wings. Plus, that frees up another $35 million for free agents.