decreasing the value of the liquid rewards (SBD) also hurts new users who don't already have lots of STEEM

Not necessarily. Rewards are determined based on both STEEM value and SBD value. If the STEEM value goes up that also benefits those receiving rewards, not only those who already own STEEM. However, to the extent that the price of STEEM goes up and the price of SBD goes down, the rewards will return closer to their intended mix of 50% SP and 50% SBD. This is likely to be more attractive to those interested in the long-term health of the platform and less in milking rewards and immediately cashing out.