You make it sound like his defense is so easily replaced. Benintendi is nowhere near the same level of defense. Really, it would have to be someone from the outside if you want to improve the position. How do you suppose the Sox acquire that player?
Internally? There isn't much. Rusney Castillo has shown himself to be a talented defender, but his offense has been sporadic in limited tries in MLB. But more important, he has also shown to be a reckless defender with a tendency to injure himself. And beyond him, the Sox don't have much else in the way of capable outfield depth, especially if your primary concern is offense.
According to Fangraphs, from 2015 to 2017, only 7 centerfielders played better defense than Bradley and his 19.3 dWAR. Of those 7, only Lorenzo Cain provided more oWAR. AJ Pollock did provide slightly less defense and a significantly larger offensive contribution, but both players also tallied exactly the same 9.4 fWAR. (While Pollock did so in 300 fewer plate appearances, the margin for improvement might still not be as much as people think.)
Now the Sox did pass on the chance to trade Bradley and his .467 OPS to the Dodgers for Yasiel Puig and his .476 OPS. Would THAT deal have made everyone happy? Are we .009 OPS points away from being satisfied?
It's not broken. Ride out his slump and realize that even if Bradley was posting an OPS over 1.400, the Sox would not be much better than 7-1 right now....