Rather than lowering the Hbd interest rate directly to 12%, it may make more sense to gradually reduce it over six months. 17%, 16%. A sudden drop in APR will greatly reduce the price of HBD.
Hbd's high interest kept the Hive price stable. People had bought Hive from central exchanges to own Hbd. When the generous Hbd interest disappears, they will convert Hbd to Hive and sell Hive on centralized exchanges. Thus, the Hive price will decrease.
This is one of the ideas, gradually lowering it to a certain point, but we'll see.
For the initial investment, you're certainly right, people did buy Hive to buy HBD (since most major exchanges are still adamant on not supporting it due to various legal reasons) so there would potentially be an initial selling spree (HBD > Hive > Sell) but I would not necessarily call that "HBD is keeping Hive alive" in itself.