HBD can be a max of 30% supply of hive market cap. For hypotetucal scenario lets say all the 30% is in savings yielding 20%. That is a max of additional 6% inflation on top of the exsiting one.
The haircut rule is what protects hive and hbd, not low apr for hbd. If we approach the debt limit, I expect the amount of hbd in the savings to drop no matter what is the apr.