In a Peterson Institute working paper, Jeromin Zettelmeyer, Eike Kreplin, and Ugo Panizza conclude that the answer to that question depends on your assumptions.

The authors compare several scenarios, including

• scenarios A–C, the baseline scenario of the European institutions and two more pessimistic variants;
• scenario I which underlies the IMF reasoning and which assumes that “Greece will not undertake the structural reforms needed to achieve higher potential growth”;
• and scenario D, which corresponds to what Greece committed to when the third program was agreed, and which represents the German position.

They assume that interest rates on privately held debt rise with the debt-to-GDP ratio, and they use two “sustainability” metrics: The debt-to-GDP ratio (should fall), and gross financing needs as a share of GDP (should be smaller than 20%).

When running Monte Carlos simulations, the authors find that for each scenario, the assumptions about growth and primary surpluses are consistent with the conclusions drawn by the different institutions:

• In A (borderline) and D, debt is “sustainable.”
• Not so in B, C, and I, due to “accelerating substitution of official debt by more expensive borrowing from private sources”.

The authors then evaluate the plausibility of the scenario assumptions. They conclude that “international evidence does not support an adjustment path that envisages a primary surplus of above 3.5 percent for more than three to four years on a continuous basis and for more than seven years on an average basis” rendering B and C the most plausible scenarios, and suggesting that the debt is “unsustainable.”

In reaching their conclusions, the authors assume that primary surpluses in Greece will react to debt, inflation, and growth in line with the experience in other (developed) economies. (This means, for example, that surpluses rise as the debt burden increases, which seems to contradict the notion of debt overhang.) This is unconvincing, of course, if one takes the view underlying scenario D which presumes that feasible promises are kept. Or stated differently: Greece might well be able but not willing to pay—after all, in this very case official creditor intervention could have made sense in the first place although private lenders charged high interest rates. (With Harris Dellas, we make this argument precise in a paper in the Journal of International Economics.) Related, one can think of many reasons why the historical experience in other countries may be uninformative for the Greek case. The authors address one concern: They focus on episodes with very high debt-to-GDP ratios and find that in these cases, primary surpluses are maintained for longer. Moreover, there is the important question of measurement: The Greek debt-to-GDP ratio is not easily comparable with the ratio in other countries, see here and here, and most likely overstated.

Zettelmeyer, Kreplin, and Panizza make the case for a delay of Greece’s return to capital markets. In the conclusions, they write that

the debt relief measures put on the table by the Eurogroup in May 2016 could be sufficient to restore debt sustainability, but only if these measures are taken to an extreme. This means accepting an extremely long maturity extension of EFSF debts. In addition, it requires either substantial additional interest rate deferrals, or locking in significantly lower funding costs and hence lower interest rates than the EFSF currently expects, or a combination of both. While these measures are feasible within the red lines described by the Eurogroup, they are likely to be politically and/or technically difficult. Unless the EFSF manages to eke out substantial extra interest relief through creative long-term funding operations, its exposure to Greece will likely have to rise, possibly for decades, before it starts falling. A private sector creditor would not accept this type of restructuring because it gives the debtor country a strong incentive to default (or at least renegotiate) when the debt is at its peak.

… one way out of this dilemma would be to delay Greece’s return to capital markets, continuing to finance Greece through ESM programs until its private sector spreads are much lower than they are now. … this approach would lower the total need for debt relief and/or fiscal effort required to restore Greece to debt sustainability. While it would lead to a significant increase in official creditor exposure to Greece—requiring perhaps €100 billion of extra ESM financing—this is less than the rise in EFSF exposure that would be required in the Eurogroup’s approach, which aims to return Greece to private capital markets in 2018 while relying mainly on EFSF maturity extensions and interest rate deferrals … total official exposure to Greece would decline faster if ESM financing were to continue than if it were to end in 2018.

Importantly, they also point to the incentive effects of debt restructuring:

If [the threat of Grexit is essential to maintain incentives for reform] keeping the sword of Grexit … would help reduce debt levels only so long as Greece is being financed with cheap official funds. If, however, Greece returns to capital markets, any beneficial incentives of this approach would likely be offset by the risk premiums that private lenders would charge to a country whose euro membership remains at risk.

The official creditors will have to make up their minds: Not only the return on their lending is at stake, but also reform in Greece.