There's a much simpler legal rationale for simply ignoring the debt ceiling than the 14th amendment. The constitution allocates both spending and borrowing power to Congress. The executive has some discretion, but largely _has_ to spend the money the budget says should be spent. So once the debt ceiling hits there are two conflicting laws. The executive must violate either the budget or the debt ceiling. There is common law precedent about how conflicting laws should be handled, and the precedent is that the more recent law takes precedence. The budget was passed after the last debt ceiling, so that's what the executive should follow.
