True; expulsion (or leave) from a monetary union will also effect the euro considerably, and those issues (mistrust, bond revaluation, etc.) can quickly evolve in an avalanche effect. No sane person in the EU wants that.
Also physically a bit tricky.
You would have to announce it in advance, to allow time for the new currency to be produced - in which time everybody would hoard euros.
Then everyone waits for the new currency to fall like a stone, so nobody uses it, so no tax revenue.