There's a couple of other factors too.
Having a 50 win team makes it very difficult to sign quality free agents next year, unless we significantly overpay them.
On the other side, assets like Donaldson and Estrada will depreciate over the year, so we'll get less value back if we trade them now.
Personally, I'm all for starting the rebuild, but I still want to do our best to put a competitive team out there, or even a team filled with young, talented guys who just need a little more seasoning. This can be easily done by trading good players away for bad contracts + minor league talent. Stick the bad contracts on the field and hope they rebound (ie Liriano), and if not we bought ourselves some decent prospects.