Downvote thumb is not for disagreements, it's for comments that don't add anything to the discussion.

Who says?

I assume at least some of the downvotes are from Eugene sockpuppets (he tends to downvote any suggestions that would make it harder to do his trolling).

As long as the definition of "senior member" includes about 90% of active members (as opposed to about 10%).

Reducing the number of voting people would be harmful, because already too few people bother to vote. (The more people would vote, the smaller impact would have one spiteful downvote. Imagine that every comment would have tens of votes, the more popular ones would have hundreds. Who would notice a single downvote?) We merely need to prevent the "hit and run" downvoting strategy.

At this moment, LW is ridiculously fragile. I mean, look at all that debate about problems caused by one mindkilled person. Imagine that tomorrow, a group of ten equally mindkilled people would discover LW and try the same strategy. (Ten people is different from ten sockpuppets; they have more brainpower and more total time.)

it's not; but it would slightly increase the effort required to do it. Eventually the effort will be high enough as a deterrent. In this case I don't think it will deter Eugene more than new users.