In an NYT Upshot piece, Neil Irwin outlined the risks that are posed to the US and world economy if Italy were to leave the euro. While the scenarios he sketches are plausible, there are two more positive scenarios worth considering.
First, it is possible that Italy is able to arrange an orderly withdrawal from the euro. In this scenario, there would presumably be some arrangement where the debt is partially written down, or there is some grace period on payments, which would amount to the same thing. This makes it easier for Italy to get through the transition period and possibly get back on a path to healthy growth.
While this seems unlikely, it is worth noting that German Finance Minister Wolfgang Schäuble proposed such an arrangement to the Greek finance minister in the spring of 2015. It doesn't seem inconceivable that they would adopt a similar view to Italy.
The other possibility is that Germany may give up its religious dogmatism on austerity and commit itself to running more expansionary policies. If Germany's economy were to grow rapidly and to have a somewhat higher rate of inflation, its increased demand for imports could provide a substantial boost to Italy and other eurozone economies.
Of course, both of these prospects seem unlikely, but it is worth noting that there are some good outcomes that could result from a government in Italy that contemplates leaving the euro.