The idea of France defaulting on /all/ of its debt is ludicrous. Ancient regime monarchies did defaults relatively often, but they never defaulted on everything. They would choose their creditors with the highest interest rates or who were the most unpopular/weakest, and default on them with some accusations of them being usurers or some other charge; a more general option was partial default on the value of debts, but again, this was partial. Alternatively, repayments would temporarily suspended in a time of crisis; the French did this in the Seven Years War and resumed payments post-war. Another favored tactic was that they would force a re-financing of their loans from short-term to long term debt, and similar to this in the French case debts above 5% interest rate would be "reformed" below this. In no case did an Ancien Regime market unilaterally default on the entirety of their creditors.
France could default in 1789, but a default would not be absolute, but rather a partial default on the lines above. Various loans above 5% interest rate would be reduced below 5%, a continuation of the suspension of payments would occur (this had started in 1787), lenders objectionable to the crown would be punished, and partial defaults on the value of debt would occur. It would cause a spike in French interest rates, and further disillusionment with public finances, but it would not be enough to seriously undermine or cripple France in of itself, no more than defaults in 1759 or 1770 did so; barely a decade after 1770, the French defeated the British after all.
Also on a note about defaults rarely being absolute, even the French Republicans never defaulted on all of the debt, I think the Directory which did a very extensive trimming of French debts still had around 1/3 of their previous debt obligations after their reforms.
Now, the political fallout of this would be extremely serious, as Louis XVI had staked a substantial amount of political capital on the idea of not defaulting, and the French government had lied about various proposals to fix the debt problem. Is it enough to cause the French revolution, in conjunction with other factors? Maybe. But it won't result in a coalition forming against France, France will manage its default to prevent too many important international fathers from being ruffled. I'm not aware of any war against a powerful nation starting because of a default, generally there is only the luxury of doing that when dealing with countries that can't fight back.
So too, the French will still be able to borrow, just as they were after the previous two defaults.