10 years ago, the bond spreads of sovereign bonds would have reflected this accurately, where Greece would have a higher YTM than Germany.

The idea of the euro is convenient, but since the crisis of 2007-2009 + various (Greek) scares, it's devolved into a "cake and eat it" situation.

A country cannot devaluate its currency (Sweden still has this freedom if it so wanted).

I'm not sure what the solution is, but to start, I would homogenize corporate tax rates, and discourage tax-shopping inside the EU.