1. Interest payments typically do not replace other government spending. This is simply not how politics works: Politicians, at least at the highest level of government, typically do not look at the budget in terms of how much is available, but in terms of how much should be spent.

2. From the perspective of the macroeconomy, interest payments are a zero-sum game. The money goes to other actors in the economy who spend it.

3. Government is always able to spend the money on programs in addition to interest payment. After all, the government runs the monetary system (I realize that this is hard to grok because it is so different from our own, private, experience with how money works, but the difference is significant.)