Whereas the breakup of the Eurozone would, first and foremost, harm Germany.

Another thing: Cameron complains about things that are actually rather trivial, no major economic consequences would ensue from their continuation. Whereas the Euro framework CANNOT continue, for the simple reason (among others) that there is no way for a sum of non-zero positive numbers to be zero.
So you can't simply reason countries into falling back in line. All you can do is blackmail them into aggravating the disaster and somewhat delaying the outcome. But status quo is not an option, it is at most an unstable transitory state.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi