I'd like to attempt an answer to your concern here.

Firstly, HBD will likely become less volatile than now, at least on the upside. When Hive will go up in price, HBD won't follow. So, a more attractive interest rate for HBD will be necessary to compensate for the lack of "price pumps", and to be competitive with other stable coins in this regard.

It happens with widely used stable coins, their interest rates are generally higher than for volatile coins.

Secondly, there was some talk in the comments above to offer an attractive interest rate for staking Hive in the savings account. If it will be done, it won't be for this HF, because it seems a bit more complicated.