The issue is about what is rewarded by the algorithm in terms of increased visibility. A downvote could act to reduce visibility, rather than as another dimension to engagement that adds to visibility. This could fix the incentive people have to post politically extreme content.

My worry with this is that it snowballs the hivemind effect.

If downvotes reduce visibility and Twitter is composed of 60% A's and 40% B's, I will expect to see vastly more A content than B content, since the like-to-dislike ratios will differ by a factor of more than 2 (over twice as much A content).

In my mind, Reddit is a prime demonstration that voting to say "I find this content objectionable" and voting to say "I disagree with this content" are often indistinguishable.