The HBD interest is not protecting the rewards pool. Oddly, the first effect of the HBD interest was to wipe out the alt-coins on Hive-Engine. One can prove this by looking at the transactions on Honey.Swap.

The primary buyers on HE were users who trade their HBD for HIVE to buy coins on HE. When the 20% interest hit, buying pressure on HE ceased and the HE coins experienced a free fall.

This reduced author rewards.

HBD is guaranteed by HIVE. When HIVE dropped into the $0.30s, many of the big HBD holders withdrew their HBD, converted it to HIVE and sold that HIVE on the open market.

The effect of high interest on HBD is to both reduce the price of coins on HE and the price of HIVE. Both actions reduce the value of HIVE rewards.

I am not an expert on whale games, but the high interest on HBD does not appear to have reduced the games.