however, there is no economic law preventing demand for HBD to exceed demand for the token acting as collateral. It could be a precarious situation if it did

I don't think this is quite true, and in fact is a good part of the reason why we want people to hold and use HBD. At some point the supply imbalance inevitably takes over. If we need to keep taking HIVE off the market to convert into HBD, then eventually either the price of HIVE has to go up or the markets simply "run out", and the latter is implausible.

What is true is that the rate of price increase as HIVE is removed from the market is not necessarily enough to offset the rate of increase of HBD, particularly given the relatively extreme 10-1 ratio for the cap.