I supported that position too. Release him in the off-season and bring in another player--the market cost for free agents was in their favor. Hanley was a sunk cost either way and you can't let that option vest.
They may have been counting on Chavis, thinking that if Hanley stinks up the joint, they would release him during the season, thus creating an opportunity for Chavis. Obviously, that plan is now dead in the water.
By the way, one of the first baseball seasons I remember was 1986. Living in New York, I remember the Mets released George Foster and replaced him with Kevin Mitchel. Of course, as we know (painfully) the Mets went on to win the W.S. that year. The Red Sox could release Hanley without it harming their W.S. chances. By all statistical measures, he is a replacement level player. I get that he might provide some leadership in the locker room, but Pedroia could take on a larger role in that regard. When Pedroia is ready to return, I'm releasing Hanley.