I just got downvoted on an answer of mine twice in a row within the same minute. Isn't that weird?
I didn't mind your answer, but I'm not sure why you would think it was weird that two people didn't like it within a minute.
Irrealis moods combined with tenses is a very advanced and difficult topic, especially when bringing up that some dialects might still use forms that are largely considered archaic, so maybe the downvoters thought you didn't explain it well enough.
Ultimately, I agree with you. "If you be good" isn't entirely obsolete, but it's odd enough to require a lot more explanation.