Suppose that Greece were to leave the euro. The best guess one can make is that the country would go through a period of hell for a year or two, after which it would have a reasonable prospect of expansion and prosperity.
Whatever might be decreed during a transitional emergency, the euro would be unlikely to disappear from the country. It would make sense for exporters and tourist operators to gain some competitive advantage by using newly devalued drachmas. But many organisations, for instance in shipping and finance, might find it convenient to continue using euros. The main difference is that the Greek government would have to finance its own budget, but would be free of German-led instructions on how to run its affairs.
Yet this has consequences beyond Greece. It might be a first step towards a new Europe in which businesses and other economic agents would have the opportunity and responsibility to decide in which currencies to denominate their transactions and hold their cash.