It's Hotel California - nary impossible to leave.

The 'good countries' that are effectively managed and where the citizens trust their own gov. more than European gov. - maybe.

But Europe will threaten to existentially punish those that leave.

Greece ... citizens would not trust their own government, and would dump national currency for flight to EU.

There would be a run on Greek banks to move their money to European banks.

In this case, the 'German straighjacket' may be preferrable.

And of course switching costs - it's massive, takes 5 years of planning and 10 years to implement.

What might work is a 'Greek Euro' - where there are some central controls and oversight but it's a national currency.

Very easily exchanged for other national Euros etc.