The market for "legitimate post promotion" is currently tiny and would continue to be tiny even if Steem grew several multiples in size. (Perhaps a few percent of the amount of rewards taken out, if that, would be my rough guess at a ceiling.) "Eventually" it could be quite large, but then the value of Steem would likely be higher and the value of extracting rewards would therefore also be higher. It does not appear clear to me there is a sound model supporting the premise that "legitimate post promotion" via reward voting would ever make sense.
Nevertheless, bots are responding to incentives. If we want behavior to change that means changing incentives. That can be a combination of financial (such as with downvotes) as well as social-layer incentives.