How can one quantify the probability that US defaults on its Treasures if there were no such event in the past?
You look at the probability of the debt being unsustainable.. For Example, if the interest payment on the debt is 2 Trillion / year and tax revenue is only 1.9 Trillion, then there's a damn good chance there's going to be a default.
S&P has default probability tables for each credit rating which gives you a sense of safety at each ranking. Of course you take this with a grain of salt because even .0001% is too high a prob of default for the US given that it would never default or only in the absolute rarest of cases, hence the risk-free moniker.
since the us can print all the currency it needs, it could hyperinflate before it defaulted (print enough money to pay debts).