So now I know those downvotes I get from time to time are because of SBI shares.
All the abuse prevention measures implemented on STEEM are based on the rewards system, so they only apply to creators (writers, witnesses, dApp founders, etc.)
Investors are immune to that measures. They can use their STEEM POWER without any penalties. If they try to get the best ROI, they can just upvote @sbd-potato articles to get big curation rewards while the author reward will be burned, and proportionately downvote random articles to modify the pool reward even more in their favour. Bidbots may be dead but there are other methods for exploiting STEEM economy.
Your idea of adding mechanics to reduce SP is a double edge sword, because it may increase even more the abuse of big downvoters. In my opinion, the problem is that SP should not be the only factor involved in STEEM governance and rewards distribution. 1000 upvotes from different accounts should worth more than 1 upvote for an account with the same SP value. However, it must be done in a way that prevents creation of massive networks of bots accounts. Maybe a 2.0 reputation algorithm?