There are no evidence-based arguments in Kevin's posts for the idea that increasing curation percentages will lead to more manual curation. There are some interesting arguments for other things, but that one is purely assertion
You can not make evidence-based arguments without trying it in this sort of system, where external behavior in response to incentives is an essential part of what determines the outcomes. The only arguments that can be made are economic structural ones, or just "what the hell, let's try it an see". Failing that the only alternative is to never change anything. The arguments put forth by @kevinwong and @trafalgar are economic structural ones. That's the best we can possibly do.
Though, in a sense there is indeed a sort of evidence-based argument in that the observation of the status quo as undesirable bordering on unacceptable is an argument for changing it.
that increasing curation percentages will lead to more manual curation
That is not the argument being made at all.
Cause he flat-out argues that? He's talked a lot about how he views Steem economics as a "race to keep up," as if other people earning more money than him is a problem that needs to be fixed.
You did not understand his argument. People are making more by behaving in a manner that directs rewards to themselves rather than helping grow Steem. That's a problem for all of us.
There are only 78 non-Steemit accounts with more Steem than Kevin
The number of accounts isn't the point. I am not convinced he has more to gain as a curator than as a poster. If anything it would have to be fairly close.
Also, you can't meaningfully count that way. My Steem is split between thousands of accounts, and I'm sure I'm not alone. I don't recall if any of them have >200K. I think so but I'm not sure and the reality is that it doesn't matter at all. Accounts are not an economically meaningful unit.
More to the point there are certainly people with millions of Steem, at least 10-20x more than what Kevin appears to have (of course he may have other accounts too). That makes his apparent holdings moderate when considering things like curation rewards to be earned and the effects of superlinear proposals.
This one you're posting with has 365k. @smooth-a has 587k. I don't know what any of the others are but the fact that you've more or less lost touch with where you're keeping almost a million Steem makes me think you should not be making decisions for people who value every last one.
I've not lost any of them, I just don't mentally keep track of their particular balances. If that bothers your sense of how the universe should work, sorry I can't help.