There is consensus on the blockchain as demonstrated when a hard fork is enabled or (as has occasionally happened in the past), not enabled, the latter indicating lack of consensus.
Other than hard forks we haven't up until this point had an objective way to measure consensus, but the SPS will do so, since it will be voted in a verifiable public way by Steem stakeholders. Previous efforts at "polls" and such things have never been objective or particularly meaningful.
I don't actually know what point you are trying to make by bringing up Steemit vs busy or esteem, but there is only one Steem blockchain, and when you are looking at things like the reward pool being paid out, it is only the blockchain that matters, not the UI. When looking at things like what is hidden or grayed out, that is UI. The issues affecting one or the other are very different.
A downvote pool is not a UI issue. It is a blockchain issue, currently to downvote a post it cost the downvoter mana, it does not matter what UI they are using. If it was a UI issue then people using busy or esteem or another UI could downvote at will if their UI did not charge a mana fee so to speak for downvotes.
There is two sides to the downvote issue if not more. At what point does needless downvoting stop? Enabling a cheaper downvote is enabling bad actors. By lowering the cost of downvoting that does nothing to curb downvote abuse as is currently happening. The vast majority of downvotes appear to be going out to comments not plagiarism, not excessive reward, but to normal everyday comments.
People talk about wanting to stop the self vote and self vote from alt accounts, start with the first issue, the self vote. Do away with it. Then see if people continue to vote themselves with alt accounts. People will see a trend pretty quickly, then the downvote for reward abuse will work.
By eventually allowing people to get paid or to pay themselves because of their downvote actions is only going to exacerbate the issue. Because getting a downvote curation reward is where this is all heading, that is pretty obvious with the desire to have a separated downvote pool.
For the last 22 months on steemit, every time the self vote is bought up, it has never been simply stopped, removed from the system. Either it is good for the blockchain, or it is bad for the blockchain. Before creating new problems fix the problem everyone keeps saying is the reason for the downvote pool and that is to stop the abusive self voters. Just do away with the self vote and be done with it.
Self-vote is neither good nor bad. It depends on the content and context.
For example, if I post something really value-adding and want to jump start the voting with a self-vote, there is nothing wrong with that, and indeed could be argued as a very legitimate reason to buy Steem for (non-abusive) self-promotion. (And indeed further argued that if I'm not willing to spend my valuable vote power on my own content, maybe I shouldn't even post it.)
On the other hand, if I post a bunch of empty comments or other worthless 'content' and self-vote them, then I am abusing the system
The only mechanism that allows the blockchain to distinguish between those two cases is having other people either agree with me and add their vote (or at least do nothing and let my vote stand), or downvote it and both reduce/prevent the payout and discourage my behavior since my vote has now been wasted.
Self-voting isn't the problem, but even if it were, it is only a small part of the problem. Paid votes are just as easy to abuse, already exist in very large numbers, and would not be stopped or even slowed down by stopping literal self-votes. So, no, that is not a solution.
As you correctly stated, downvotes have a high cost and as a result there are only a tiny number of downvotes that happen (and yes, as you say, many if not most of that tiny number of downvotes are from annoying jerks), and so naturally we have people abusing the system though self votes and paid votes all day long.
I disagree with your suggestion that the most effective way to address the core problem is by focusing on literal self-votes. Not only is it easy to evade, but even without any obstacle in place to evade it is already only a narrow subset of the problem, effectively a distraction.
Probably never. Any system which allows people to interact will always have jerks who want to annoy others. But it also should not be the case that what one or two jerks do matters much at all (or, indeed, the system is much too fragile).
So, again, like I said earlier, probably UIs should demand a lot more downvoting before paying attention to it and performing hiding, negative reputations, etc. (effectively ignoring the few jerks who can still downvote, but will not be able to meet this threshold very often if at all). And for that to happen, we need a lot more downvoting from ordinary non-jerks, where well-deserved, which in turn requires that good non-jerks not have to burn their valuable vote power in order to perform a public service.