a submission can be both against the rulesLock the thread (with AutoModerator) and explain why. No need to disappear it.
Over time, people submitting good content stop doing so, and people who submit bad content start doing it more. Vicious circle.
Is this really the case? It's a common complaint, but it's usually presented without any backing. Why is the content necessarily "bad" if the community has chosen to feature it?
Mods can't just delete those posts either, because this reddit gets pissy that they're playing favorites.
Why do you suppose that is?
It would be better if the mods were able to say "if you think this is upvoteworthy for this subreddit, you are no longer welcome here".
Leaving aside the fact that you literally just advocated for thought crime, the problem is that an upvote can mean multiple things.
(Don't say "reddiqutte" either, I'm talking about use in reality)
An upvote can mean "like", or "on topic", or "contributes to the discussion", or about 20 other positive things. A downvote can have just as many other meanings in the other direction. The other thing is that with multireddits and /r/all, "on-topic-ness" isn't even going to be evaluated by the users unless you're looking at the front page of the community itself.
I really don't see how more covert moderation is a valid answer to abuse of non-covert moderation.