I thought Dmytry sometimes has interesting ideas, and it'd be worth trying to convince him to stick around but be more careful and less adversarial. As orthonormal said, LW needs better contrarians, and Dmytry seems like one of the more promising candidates.

There is some potential there - Dmytry has what seems to be a decent IQ and some technical knowledge in there somewhere. But the indications suggest that he has more potential to be destructive than useful. I would expect him to end up as a XiXiDu only far more powerful (more intelligent and rhetorically proficient) and far more hostile (XiXiDu's attitude hovers just on the border, Dmytry given time would be more consistently hostile).

Why tell him to go away?

His idea, I merely agree that it would benefit him and us. For what it is worth I don't think my agreement is likely to encourage him to leave. If anything he would be inclined to do the opposite of what my preference is.

In terms of my own personal interests - I incur a cost when there are people like Dmytry around. My nature (and considered, self-endorsed nature at that) is such that when I see people try to intellectually bully others with disingenuous non-sequiturs and straw men I am naturally inclined to interfere. Dmytry is far from the worst I've seen in this regard but he's not too far down the list.

If the guy wants to leave and has concluded we are too toxic for him then I'm not going to argue with that. It seems better for everyone. Arrogant nerds are a dime a dozen - we have plenty around here so don't need another. And communities where one can show off technical competence and rhetorical flair are a dime a dozen too so Dmytry doesn't need us. I'd recommend he try MENSA. He would fit in well (based on what I recall of my time there and what I have seen of Dmytry.)

Do you think my effort was doomed or counterproductive?

Doomed.