Koji may not be 1.09 ERA good next year, but he's still good for a 1.9-2.2 ERA. His ERA for the past 3 years is 1.69. This wasn't some kind of aberration of a year for him.
As for the entire pen, though, having Koji, Taz, Miller, Breslow, Dempster, and a slew of prospects is a very good base. I think the Sox pen will be good again this year. Can't depend on Bailey for anything but he could provide some additional depth as well.
Tack on a couple guys like Crain, maybe LaTroy Hawkins, etc, and you can have a really strong pen.