If a high reputation account turns malicious then there will only be a small number of accounts that can make an effective downvote, that is true. There are strong incentives to discourage malicious behavior, though. The people controlling high reputation accounts have a lot at stake, both money and social reputation, that they can lose if they misbehave. The whole design of the Steemit platform and the STEEM token is intended to create and incentive system to reward constructive social interactions and discourage behavior that harms the platform.