On the other hand, if miracle happens and someone will be dedicated enough to make an effort to legitimately moderate bad quality posts of a single tag, then not only he doesn't get deserved praise for his hard work but instantly becomes marked as suspicious person, enough to make one of mods to raise an alarm on the forum.
I honestly believe they should. It's just another kind of lone wolf behavior with all the personal baggage that applies to the "spammy" contributors. Doing the same thing but on the opposite pole doesn't make someone better, and danbooru subjectively should, if anything, err towards having something rather than nor having it, in my opinion. So I would prefer having spammy contributors and not having spammy flaggers to the opposite situation. However I would still prefer to not have either.
Does it somehow risk making danbooru too slow if you get rid of spammy contributors? Well, there are other ways to deal with the "problem". It's not black and white. Just like I'm okay with spammy flagging as long as I happen agree with every single flag (despite not liking the idea), I'm just as okay with spammy contributors as long as they don't get weird.
If something is being "spammed" by a contributor, it should be much easier to have a short conversation about it and have a couple people chip in. That way it's much less likely for other people on the site chipping in to reason about it to become toxic, and it's much less likely for one person's personal taste to affect the way in which they go after the "spammy" uploads.
In any case, this is turning into a different topic, and it's quite the bikeshed, so I'll stop posting.