By suggesting that a road not taken should be taken, he's committed the sin of being a possible source of regret. Without him, Greece could just pretend their situation since is due to the weather.

If you're an ideologue, you can somehow twist that regret into blaming him for German banks making massive loans to Greece to pay for things like military equipment that they didn't need, and the inability of Greece's currency to adjust in order to keep this from happening. That's a lot accomplished for an economist who was mostly ignored, except by the voters who his party ultimately ignored.