Investors should take a substantial loss.
Well that's a real problem because the "investors" that would take the "haircut" are the banks that took the LTRO money from the ECB and to a lesser degree the IMF.
Do you wonder why interest rates are on the rise again? Ask the head of the Bundesbank: http://www.bloomberg.com/news/2012-04-22/bundesbank-s-weidmann-says-what-no-eu-politician-wants-to-hear.html
Of course when the haircut is taken the banking system in Europe will collapse and they will fall into a full blown depression. What private sector investors learn in Greece, they now practice, don't buy European Sovereign Debt. So there's really no secondary market for those pieces of paper.
Ultimately that haircut will be taken from the population in Europe.