It's still pretty dumb to pull that so young though. Jay-Z was what, in his mid-30s at the time? Did he really think he'd be able to go the rest of his life without making any music at all? What was he planning on doing for the rest of his life? Go golfing every day on "99 Problems" royalty checks? Did people actually take him seriously at the time?
A band situation on the other hand can actually end which means that grand proclamations of their demise can actually hold weight. James Murphy could have easily just started up a solo act if he wanted to continue making music.