Should we just codify it into the system that NO ONE can self-upvote "ever"?

Oh wait, what about our botty friends who will upvote their master instead of self-upvoting him or herself?

Seems like you answered your own question why that rule shouldn't be codified into the system. I don't support any change to consensus that doesn't pass the Sybil test. Unfortunately, this is something many Steemians have yet to learn, since it seems like 99% of the changes I see requested on Steem by regular users don't pass the Sybil test.

The excessive self-upvoting problem is a difficult problem to solve that really needs to be left to human beings to try to find and deal with (via downvotes). It is more important than ever because of the linear rewards change, which I hope will provide more benefits than the abuse downsides, but we will have to wait and see how it turns out in practice. To help with this issue, we should try to avoid associating shame with downvoting (aka flagging) blatant self-voters who regularly give themselves too high of a reward (too high is of course subjective).

Downvoting is a healthy and necessary part of the Steem system to keep people in check and fairly allocate rewards to those who produce valued content. It would help if the UI separated out displaying downvotes for the purposes of too high rewards from the more menacing flag which could be reserved as a symbol for downvotes given for reasons involving fraud, scams, plagiarism, and/or hate speech.

Edit: Also, thank you Hive account@jesta for ChainBB which allows me to upvote with less than 1% voting weight. Although, it seems its estimated rewards calculation is a bit off.