What do you believe to be the limit on HBD interest rate? If 20% is better than 12%, why not 30%, 50%, 100%, 1000%?

HBD interest does come at a cost to Hive holders, it both directly increases Hive inflation and it encourages risks related to Hive inflation. At the moment high interests are likely acceptable due to low overall savings rates, but there are ways that could quickly change.