The primary purpose of tags is to connect experts with interesting questions to answer.
The question "should tag A be merged in tag B?" can be rephrased as "can a person be an expert on topic A but not on topic B?". If the answer to that is "no", then the tags should be merged.
In this case, I don't think anyone can be an expert on "debt ceilings" but not be an expert on "debt", so for the purpose of tags they are equivalent, and should thus be merged.
Since debt-ceiling has only three tags I would just manually retag them.