Is the Euro a sustainable currency?

Nail Ferguson brings it up in the context of bashing Krugman.
I am not the expert here, and tend to waver. But my null hypothesis is why not? Banking should be liquid enough.

The counter argument is that a government, currency and private sector should match; they should all have comparable yield curves. Then when external terms of trade change the fortunes of the economy the local fiat banker has seigniorage and can buy up weakened assets, essentially lowering rates to match the lower growth of the economy.

But, then, in a common currency the bankers should be monitoring the terms for trade of its members, they don't. With separate currencies the retail goods traders are responsible for the cost of currency rate changes. So, using a common currency, bankers take up the task monitoring terms of trade.  Do their job and charge a fee.  My null hypothesis stands; a common currency works  if bankers do their job.