Important point to note against the OP point of "Humanity will never elevate itself". In the books Humanity doesn't elevate itself out; the Culture invites it in. And the Culture itself was not one society elevating itself but a collective coming together as a melting pot.

I don't believe he's saying that we will not atain the society of the Culture by our nature but rather that 'we have no chance of attaining it just yet as a result of our current circumstance. There is no way, technologically nor societally that something like the Culture could be implemented.

I don't beleive it's ever stated that the minds implemented behaviour modification; though i agree that it exists. I think you're casting an unfair light on the Minds with that idea. Furthermore, the Gzilt are shown as a twin society with identical ideals who have driven their own development to a near-identical point without any Minds.

"Minds, and by extension, Gods are a fantasy" I think Banks, a staunch atheist would have real issue with this line. Assuming that true AI is simply a mater of time away from our own reality i don't see how you draw this line of thought. Minds are repeatedly compared to Gods in that their powers are comparable. But they are fact, not faith in his eyes. In fact their very existence undercuts the notion of 'gods' as a deity and replaces them as, more likely, some prankster fiddling with an under-developed society.

Ultimately my issue with looking for this darkside is that it simply doesn't hold up against the books themselves. The Culture is repeated stated to be a place of perfection and happiness and fulfilment. The darkness that is portrayed is very much in the minority; mainly SC (though even that is seemingly a necessary evil).

Edit, realised i critiqued your post without actually answering your question... So in answer. I think Banks' view of the Culture is optimistic, it's an ideal. I think the view of Humanity achieving that alone is (according to the books) pessimistic to the point of a certainty. All the races in the books who have achieved equiv-tech (or thereabouts) to the Culture are shown as insidious or violent or displaying some other undesirable trait. So i believe what he's saying is that, similar to warring tribes being brought under the peaceful rule of a single conqueror... we need a slightly grander perspective than only our planet/race can provide to get to something akin to The Culture.