It's a simple fact that superlinear curves reward concentration, as you say. And naturally this can be exploited. The obvious exploitation method is pre-agreements of one sort or another to vote on the same posts in a way that generates reciprocal shares of the resulting rewards over time. If a sufficient amount of stake becomes involved in such arrangements, then the reward system by itself no longer incentivizes upvoting based on content.
Note that this doesn't necessarily guarantee bad curation by such groups. The people acting in such group do still have a reasonable financial motivate for good curation if we assume that good curation makes the coin more attractive to holders. If the group is curating badly, I think it's reasonable that competing groups will arise that "do a better job".
And I think it's also reasonable to assume that a certain amount of people will decide that they would rather vote the way they like, rather than join such a group, in which case they will lessen the rewards of joining such groups.
I don't see how the mere existence of cheaper downvoting necessarily changes the economics above. You've already described it as "altruistic/community-motivated" downvoting. So altruism or like-mindendess is already assumed as a force that exists on the platform (I agree with this assessment, btw).
But altruistic "voting your conscience" also redistributes the rewards, just like downvoting does. It's true that voting "can" be directed towards one's own posts, but if it's altruistic, it certainly doesn't have to be.
The best argument for downvoting from a curation perspective is only that it allows you to actively distribute the rewards away from a particularly bad post that is being upvoted for selfish reasons.
In a system with lots of voters and posts and with a more distributed stake, I'm not sure downvoting would have much benefit at all.
I think in such system, the type of bad "self-voted" post I'm talking about would never get enough rshares behind it to get much value. The only way I could see it happening would be the rise of a mutual-voting group of the type I described at the beginning and the only real counter for such groups is a sufficient group of people who vote differently.
The difference is that this has a huge opportunity cost compared to voting to maximize your own reward (either directly or via some scheme). Since the cost is very high, it is unlikely to see such altruism persist. Sure, not everyone will maximize their own reward, but when the incentives are misaligned in this manner you expect the economy to shift more and more in that direction over time, which is exactly what has happened. I don't believe this has anything to do with stake distribution or the number of voters or any such thing. It is more that it is baked into economic design.
By contrast, once the high cost of downvotes themselves is removed, good downvoting vs. no downvoting vs. bad downvoting does not have a high opportunity cost (in effect virtually none), so it is far more likely that altrusim will succeed. If people had to pay money (or forgo a significant payment) every time they edited wikipedia, hardly anyone would ever do it (in fact, those who did would be more likely to be promoting some sort of manipulative agenda and often damaging the end product). But because editing wikipedia is very cheap/free and frictionless, you do get altruism on a large scale. Crowdsourcing works, but not if you expect people to pay a lot to participate. The only way to get this with Steem is by crowdsourcing downvotes.
This not just a question of downvoting being used actively to 'fight abuse', which is the traditional way we have looked at it. It is a profoundly different setup of the incentives, which can't be accomplished by upvoting, because the economic 'pull' of voting to direct rewards toward oneself and/or one or more collaborators is always there with upvotes. With downvotes it is not.
Such groups would certainly arise if there were significant money at stake, probably cleverly packaged into a game, challenge, service, meme, corporation, investment fund, etc. That is the nature of human creativity.
In any case, we can't simply wish ourselves into a system with evenly distributed stake and billions of users either, even if that would somehow manage to work. We have to need to have economic rules which are robust to real world conditions. Which means downvotes.
Well, no. A superlinear reward system never, in and of itself, incentivizes upvoting based on content. Structurally it only incentivizes concentration. And concentration that rewards the voter is incentivized most of all. Content is actually irrelevant unless there is a large enough portion of the stake voting on an altrustic basis, and as I've argued above, it is very unlikely to see altruistic upvoters persist on a scaleable, sustained basis. (Things might start out that way, but they will surely devolve over time.) The only real hope we have is altruistic downvoters.
Please give this some more thought. It took me the better part of two years to finally reach the conclusion that downvotes are essential (and not just theoretically-possible expensive downvotes that are hardly ever used, the way we have now), but I'm now quite confident it is correct.