I actually think this wasn't a "big" trade from any perspective, even though it's seen as a "major" trade for the eventual WS victor. Success may have 1000 fathers, but in this context Peavy's paternity suit is weak.
Take a look at Peavy's game logs:
http://www.baseball-reference.com/players/gl.cgi?id=peavyja01&t=p&year=
He was good in Au.g (6 starts), meh in Sept. (4 starts). In the post season he gave us 5 good innings against the Rays, imploded in 3 against Detroit, and gave us a mediocre 4 against St. Louis.
He may have pushed Doubront/Workman back into the pen, but there's no overwhelming evidence he was a better option than either of them (meaning, if we traded for Peavy and stuck him in the pen, the outcomes may well have been the same).
So we got some rotation insurance/depth, but it's not like Peavy was driving the car (apart from a couple of excellent starts in Aug.)
Peavy's future value to us isn't outstanding.
***
Iggy's lost value to us wasn't great (for 2013) - he later hit the wall with Detroit and became a mediocre hitting excellent defensive player. It's fairly safe to say that the Drew/WMB/X combo gave us more value than Iggy would have.
Iggy's future value is speculative. Right now Iggy isn't good enough to start for the Sox at 2b/3b/SS, given Pedroia, WMB, X. Granted, he'd be great defensive depth. He'd also be immediately pushed out of that depth if we resigned Drew and wanted to spell X between SS and 3B.
***
Sure, in alternate universes, we do the trade, Lester gets hit by a come backer, and Peavy somehow carries us to victory.
Or we don't do the trade, our pitching staff is fine, but Drew gets a freak injury, and Iggy stays hot through the end of the WS.
Or we don't do the trade and the pitching staff gets the yips and we miss the playoffs.
Whatever.
I feel the same ambivalence about each player's future prospects - maybe Iggy will tweak his swing, maybe Peavy will tweak (or retweak) his arm slot.
Regardless, I'm 100% fine with the trade, no matter what happens to these guys in 2014.