I don't agree with your statement about a progressive income tax being economically unjustifiable, especially in the economy that we find ourselves in at the moment (and, if secular stagnation proponents like Larry Summers and Paul Krugman are right, may be the structural norm these days). The current economy is strongly demand constrained, with plenty of money available for investment (corporate profits and cash holdings at record highs), but inadequate consumer demand to justify investments. From this perspective, a progressive transfer of wealth down the income scale makes a lot of sense economically because the poor are much more likely to actually spend that money than the rich are. There's also not much good evidence that high end tax rates affect macroeconomic growth as much as people claim - just in the US we've had tax rates all over the map over the past 100 years, and there's not much correlation with economic growth. We certainly haven't seen any miracles of economic growth for the economy as a whole since 1980 with the long running supply side, deregulation, and low tax experiment. If taxing the rich really has such a strong effect on economic growth and job creation, it's hard to see that in the data...
Finally, interest on government debt only becomes a long-term problem if the rate is higher than the rate of growth. It's not clear to me that that has to be the case, but that's another discussion ;)