And this is where your argument fails. The Sox brass did identify Iggy as a startable shortstop, in the sense that he was someone you could start in short without hurting your team. What he isn't, wasn't, and never will be is the kind of superstar you bnuild a team around. Bogaerts? Is. Or at least is beginning to look like that kind of player and already showing a performance level that's impressive enough to dream on.
Iglesias was traded exactly BECAUSE the Sox brass realized that Iglesias was good enough to start, and that he wouldn't be starting anytime soon in Boston. When you can either move someone into a position where they lose the greater part of their value as a player, OR you can make a trade, the right move is often to make a trade. All teams do this, and all fanbases occasially tie themselves in knots with the woulda-coulda-shoulda game when they do. It's nothing new under the sun.
Anyway, all that needed to happen for the Iglesias trade to reflect well on Cherington is to NOT get as literally unlukcly as mathematically possible on at least one of Middlebrooks, Cecchini and Sandoval. If at least one of those 3 players chooses not to backfire spectacularly, the Iglesias trade would have been the exact right decision.
All a GM can do is play the odds, and the odds looked like the team had plenty of resources to solve 3B without moving the best Red Sox prospect since Jon Lester off his best position.
if events are sufficiently determined to go against him, that all 3 of the reasonable depth measures he had available to fix 3B at the time are going to backfire on him exactly as spectacularly as possible, there's simply only so much a GM can do about that. How many layers of redundancy does a team need before it's OK to take smart risks that are aimed at improving our weaknesses so we can win World Series? With championships in play (and they are in play now, and definitely were in 2013),One must not be paralyzed by maybes, and that's exactly what not trading Iglesias would have been.