Even that's no guarantee; the law can be changed just as easily as it was passed.Assuming a debt of the United States would be paid back is just that: An assumption. It is not written in the Constitution that Congress has to pay it back. If Congress doesn't pay its debts, the credit of the United States goes in the toilet and there is no credit for the United States to borrow against. Any debt you or I incur and don't pay back will ruin our credit as well.
Article I, Section 8, Clause 1, authorizes Congress to collect taxes for, among other things, to pay the debts of the United States as incurred by Article I, Section 8, Clause 2. Congress has direct authorization to borrow money. How and when - if ever - those authorized debts are paid back is not mandated by anything in the Constitution.
The exercise of the powers granted to Congress are not mandates - exercise of those powers are discretionary. For example, Congress has the power to declare war, but it doesn't have to. If it were to say that Congress SHALL pay the debts of the United States, that would be different.
If you want to loan money to the United States, make sure you get it under law as to when and how you are to be paid back, and with what interest.
Woody