arcange's comments are mostly on point as to the downsides of the suggested approach. Also, it's important to understand the the use case for HBD goes far beyond just the proposal system. Other forms of commerce also need a stable value, just like proposal payments.

The solution you're suggesting requires additional coding (for example, for the HBD feed price) and it only would solve the stability issue for proposals. The planned solution is to stabilize the price of HBD itself, so it can be used as a stable currency for many types of commerce, and it is actually easier to implement and maintain.