Does the word "You" actually require a different plural form? If you think of the practical usage of it, to address a group or an audience "You" is just sufficient, isn't it? Because, if you want to call one or a few specific people of that audience, you would rather use his/her name or a phrase such as "you, sir/madam". Otherwise, it naturally addresses the entire audience/group when you just use "You". If you think of the natural cognitive impulse that a common person (without being region specific) would get when his group being called with "You all"; it is more like issuing a command (not that polite) isn't it?