what exactly can you achieve by such a blatantly obvious behaviour

If I remember correctly, discouragement of "wrongthinkers" is Eugine's explicit goal.

One possible outcome is discouragement of Elo (and other users who get downvoted regularly). Other possible outcome is discouragement of people who would like to express opinions different from Eugine, but don't want to get involved in the ongoing karma manipulation, so they either avoid the topic, or leave the website. Both outcomes seem compatible with what Eugine wants.

In the spirit of "trivial inconveniences", making people annoyed -- even by something pointless -- can cause significant differences in their behavior. The changes on LW are mostly not about what people, but what people avoid doing.

Eugine failed to make LW a recruitment ground for his cult, but he can at least enjoy watching it burn.

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(Disclaimer: I actually didn't like the few recent Elo's articles. But that's of course different from going and downvoting every single comment he made.)