Your closer, like any other reliever on your roster, needs to get outs. If they get outs, they succeed. That's it.
I never said Daniel Bard is a better reliever than Papelbon.... yet. Next year, it could ultimately turn out that way if Bard is given the chances. Papelbon has seen a decline this year. There's no guarantee he'll bounce back; he could very well continue to decline. It is much better to trade him too early than too late. If you ship him out now, there's bound to be teams out there who see this season as a bump in the road, and are willing to part with something valuable for him (the Marlins traded Kevin f***ing Gregg for Jose Ceda, a good relief prospect).
Also, IIRC, Papelbon wasn't even slated to be a closer heading into 2006. Francona just threw him into one of the Rangers games, he did a good job, and it stuck. Why can't Bard come in and do the same?