It doesn't presuppose. It asks the question: what is a sustainable public debt? Then, it goes on to answer that question. The paper referenced reaches the concl

It doesn't presuppose. It asks the question: what is a sustainable public debt? Then, it goes on to answer that question. The paper referenced reaches the conclusion that three different models used to analyze debt sustainability raise serious questions as to whether today's public debt is sustainable. They go on to suggest that, in the US at least, taxes on labor can go a long way towards making the debt sustainable, while in Europe, neither taxes on labor nor taxes on capital will do much to make the public debt sustainable (since both are already high in Europe). Additionally, taxes on consumption can help in the US just as much as taxes on labor.