My understanding of the change to self-voting with HF20 was that the author would no longer get 100% of the curation reward of his upvote when upvoting immediately. Instead, that portion would be returned to the reward pool. This is a relatively modest change that affects some of the curation rewards for an upvote, not the value of the upvote itself and certainly won't prevent self upvoting and rewarding ones self. Unless I'm misunderstanding something.

From https://steemit.com/steem/@steemitblog/steem-velocity-hardfork-hardfork-20

According to the current blockchain rules, if authors vote for themselves immediately, they get their author rewards, 100% of the curation rewards from their vote, and a portion of the curation rewards coming from everyone who votes for the post after them. Any other curator voting at the same time as the author would get 0% of the curation rewards. This gives the author an unfair advantage over other curators, because the author can earn additional curation rewards through self-voting.

In order to eliminate this advantage, the portion of curation rewards that is not given to the curators due to the early voting penalty will be returned to the rewards pool instead of being awarded to the author, thereby increasing the overall percentage of rewards paid to curators. This will better serve the original mission of the curation rewards budget: to ensure that the Steem blockchain distributes rewards to the most valuable content.