At that point, i. e. when a primary surplus is achieved, things will get tough because at that point, for the first time in years, Greece (and its creditors) will have to choose between options. One option is to live by agreements and to use any primary surplus for debt service. The other option is to use the surplus for other things.
So far, the debt service was immaterial to Greece. Whatever interest the creditors demanded, they had to first lend that money to Greece before such interest could be paid (to themselves). While that increased Greece’s debt, that is sort of a moot point because it seems certain that much of that debt will have to be forgiven sooner or later. With a primary surplus, the Greek government will, for the first time, be in a position where it has to explain to their electorate why they used excess cash to pay interest instead of doing better things with it.
If and when the primary surplus is reached, Greece will have to decide whether to use it to pay interest or perhaps to invest in the economy. Should Greece decide not to use it for interest, major problems with creditors can be expected because that would violate agreements. However, perhaps the creditors will come around and see the light in the sense that it is not prudent to drain cash from a weak borrower. In restructurings, surplus cash should never be used to pay dividends. Instead, it should be used to create even more surplus cash going forward so that even more dividends can be paid in the future. The expression for that is ‘investment’.
So far, an interest moratorium would have had no major impact on Greece because all it would have meant is that creditors would have had to lend less money to pay (themselves) interest. With a primary surplus, an interest moratorium starts making eminent sense.
In conclusion: as Greece is headed towards a primary surplus, very serious thought should be given (not only by Greece; also – and foremost – by its creditors) to the optimal use of that surplus. I have proposed a solution once before: transfer the primary surplus into an escrow account which is targeted for new investments.