In my numerous dialogues with @themarkymark about this issue (which we clearly disagree on, but about which we have been able to continue constructively dialoguing), he has stated numerous times that he basically views my concerns as a 'non-issue' (i.e. seeking a solution to a non-existent problem), that downvote abuse is rare, and that the benefits of free downvotes far outweigh the downsides.

Not as much of a non-issue just a extremely small and very rare issue. There is 10,000x more upvote abuse happening on a daily basis. Yet almost no one does anything about it.

"Downvote abuse" which happens very infrequently already has a very simple solution. Upvotes! Problem solved. Post rewards are a community effort and no one person has complete say on them. If you disagree with the rewards for a post (too much or too little) you can do something about it. The real problem is rarely does anyone.

Either way, I hear about one or two people being “maliciously downvoted” every 3 months or so. I’m downvoting garbage upvotes on a daily basis for the last 4 years.