They could have 10 - making 30%! How would we know?
30% is actually a lot. Most of the largest account owners are known. But even if it were it wouldn't matter. 30% is still better than 100% and a 30% stakeholder who can gain a 30% benefit from part of the reward pool does not accrue any advantage out of balance with stake (in fact likely much smaller).
enhanced downvotes increases the gap between the rich and the poor
It doesn't really. In the hypothetical that everyone is participating in the reward pool, the immediate gain is less than proportional to stake because you can't downvote everything and you only gain back your stake share of the portion that you do downvote. There is no disproportionate advantage to being rich here. And ultimately there are still opinions being expressed and adding value to the system that way (unlike self-upvotes).
The amount of content involved means that this can ONLY practically be done by a decentralised method - such as with voter muting or something similar.
This is exactly why we need downvotes to be widely used by people expressing many individual opinions in a decentralized network. Smaller accounts can downvote smaller and/or more obscure overvalued content, or can just pile together and do their part. Larger account holders may focus on larger rewards.
the actual downvoter does not personally benefit in a way that is out of balance with everyone else
The actual downvoter does not gain out of balance with everyone else given the status quo of rewards going back to the pool. That's the whole point. They gain the same as everyone else (in the pool) except the one item of content they happened to downvote. This is about the same really. In the case where the the diversion is "meant to benefit steem", the person who owns 3% still gains more than the person who owns 0.1%, and the person who got downvoted loses relative to anyone else. There is no difference!
In the case of a charity that doesn't benefit steem at all, maybe, but then you have other issues of who chooses the charity and whether that choice actually benefits someone somehow.
Nothing is perfect but all of everything both of us has written still demonstrates is that downvotes are still far less exploitable for personal gain than upvotes and far more aligned with expressing opinons and proof of brain.
Voter muting or any other sort of analysis of data intended to recommend content can certainly be used at the UI level but has no relevance to rewards or proof of brain. You don't even need a blockchain to recommend content at all for that matter.
Also, your example of a niche does not work as long as anyone else actually cares about the niche (if not then the discussion is pointless). The downvoter's own content being overvalued will attract downvotes and other downvoted, now-undervalued content will attract upvotes. One single downvote (or upvote) does not end the game, it invites others to express their opinions too.
More broadly, you shouldn't assume that the goal is for stakeholders to not have influence. If a large stakeholder has a strong opinion over some niche or even that some group of new users should be downvoted, it isn't improper for that opinion to matter a lot, even potentially to the point of monopoly (until and unless more stake takes an interest in the matter). It is still an opinion after all and proof-of-brain does not mean proof that others think the same way that you or I do. The goal in fact is absolutely for stakeholders (including large ones) to have influence but for it to be expressed in the form of opinion over reward allocation (even if, again, you or I do not like the outcome) rather than purely self-enrichment.
As already highlighted, there is more involved here than pure money
The money component has to work at a broad level first. Given that proof-of-brain and incentives to grow and create value via voter-allocated inflation are the flagship features which differentiate Steem in a crowded market, it is imperative that those work.
Once that is on solid ground we can talk about softer goals likes free speech, unpopular viewpoints and such. Free speech isn't even an issue here unless you believe people have a right to have others carry the cost of their speech, and I disagree.