If someone downvotes based on a subjective opinion on a post's value, it should cost them voting power and not give them anything back.

How and why does this differ from upvoting based on a subjective opinion?

IMO the adjustable voting power thing isn't that important, even for whales, because the system already adjusts your vote power to how much you vote. Whales might fine tune this a bit, but the incentives favor, overall, using as much of your vote power as possible, which is close to what the system already does.