Conversion has a fee of 5% so if a someone were to try to obtain a very large amount of HBD (higher than normal emission), the blockchain would make a large profit on the transaction, and besides the amount of HIVE removed from the market for the conversion itself would likely result in a large price increase. There would then be no guarantee that the current high interest rate would continue, because after all the point of it is to encourage people to hold a healthy amount of HBD. If there is too much demand then the interest rate is too high. So the person buying massive amounts of HBD quickly might be paying that 5% fee for nothing.

I believe the system as it is set up can probably deal with keeping demand healthy and in balance, but if not we'll have to consider other modifications.