What happens if someone with a large amount of HP, like @blocktrades and @blocktrades.com, says, "Vote for our proposal, or we'll downvote your proposal and guarantee you'll never get the 28.1 million HP to pass your proposal"?

It doesn't have to be existing accounts with Hive Power. Perhaps just someone who won't invest in HBD savings because "a 20% APR for inactive money isn't fair," but 10 million active HP downvoting would be active and thus "fair."

You don't need a downvote