Selfishly, I would appreciate it if HN would adopt downvote reasons because Lobsters has to acculturate people used to how HN does things. Occasionally they're outraged that we do not have downvote-to-disagree when a mod contacts them to ask that they stop picking random reasons to do so. If HN would be so kind as to train several million programmers in the UI and norms I'd appreciate it.
Less tongue-in-cheek, I do think flag/downvote reasons are worth the added complexity for us, but as there's a cultural component to these design issues it's probably a non-starter for HN to adopt. A change in such a fundamental interaction on the site would be an upheaval that's not justified by probability and size of an expected benefit.
+1 on basically your entire message, especially recognizing the limits of mods and size as dominant variable on roughly everything to do with communities. Best of luck.