A good model to think about the indebtedness of France:
I keep being asked. How long can France go on with budget deficits and increasing debt until we get a crisis? The answer: I cannot tell. Not because I am particularly dumb (this is a separate issue). But because there is a potentially large danger zone, where the crisis may or may not happen, depending on the whims of investors. Given our debt and our deficit, we have probably entered the danger zone.
If investors continue to be relatively relaxed, they require only a small risk spread, the interest bill may not be too large, and debt, while high, may be sustainable. But, if for some reason, they worry and require a large spread, the interest bill explodes, debt becomes unsustainable, and this triggers a crisis. To be nerdy about it: both equilibria are rational equilibria. Investors beliefs are confirmed in both cases.