IMO you are obsessing over something unimportant and uninteresting.
Everyone gets 2.5 free downvotes per day and is entitled to his or her own opinion on how to use them.
My opinion was to use it here, yours is not. That's really all there is to it.
But again, I don’t see a problem for good post like this getting more reward and the Trending placement is utmost deserving
Vote buying is itself a systemic risk. It's marginally worse if done on bad posts but still bad no matter what. If you don't agree, that's fine (although it indicates some lack of understanding of the economics), use or don't use your downvotes how you see fit.