The problem is really that very few people actually read what they upvote so Proof of Brain is just a phrase it doesn't happen. Why do I say this? Because a lot of the bigger accounts vote at 10% or even less of their vote value that means 100 posts a day, I can bet you nobody on Steemit takes the time to read 100 posts a day so goodbye Proof of Brain. And now with downvotes if one of the bigger accounts gives a big 100% $5.00 or $10.00 upvote the chances are high that you will be targeted for downvotes,so see, there is no easy solution.