Because that’s how decisions have to be thought through the in real (baseball) world. The Cubs got a few decent years out of Alfonzo Soriano (and 2, brief, playoff appearances), but are now rueing the decision to sign him for all those years (as will the Nats vis a vis Werth). You simply don’t need to be throwing out tons of money and seasons to 30+ y/o players who aren’t likely to be worth much during the second half of their contracts. And in VMart’s case I’m not even convinced he’ll be all that helpful in the short term. Gonzalez is young enough that they probably won’t regret the trade (and assumed multi-year deal)-maybe. It’s true that you still have to risk such signings sooner or later-Manny never repeated his Cleveland peak (mostly because his average glove became horrid), but the Sox probably would redo that deal again anyway, knowing what they know.
In basketball, I recall the 76ers offhandedly trading a fringe player (Kobe Bryant’s dad, FWIW) to another team for a draft pick 7 years in the future. Sports Illustrated at the time called it “worthless.” That draft pick turned out to be Charles Barkley. But to your way of thinking nobody should ever do deals like that simply because it hurts the team in the short run.
While the SS revolving door is an admitted weakness (mainly brought on by Nomar’s quick decline-he’s the same age as Jeter but ceased being a championship quality regular after 2003) and Theo’s made some strange decisions there (signing then quickly dumping Renteria, letting AGon walk then picking him back up 2 years later), they did get Ellsbury and Lowrie for Orlando Cabrera. Keeping any of those guys probably wouldn’t have been better than any other alternatives (except for <shudder> Lugo). Even successful teams are probably going to have a position where they can’t find someone decent (see Yankees and CF after Bernie Williams or even during his last few years). There just aren’t very many quality SS’s around anymore,and the Hanley dead was a pretty good one for Boston all told-the Sox have continued to win anyway.