It's true! But it's also imprecise because of that ambiguity. Take the following construction, "Now, all you guys are going to step off the dance floor."
Which could mean everyone clear out, or just the fellahs.
I don't get worried about people preferring a term that isn't "you guys", because it's probably an improvement to the language over all, even if it's some friction to change.
I don't think anyone would use that construction though in the midwest if they only meant the men. If they wanted all the guys to get off the dance floor they would say "Now, all the guys are going to step off the dance floor." "The guys" is much different than "you guys", at least where I live.
Granted, it is murky so I also don't really care about switching to y'all. The only problem I have with "y'all" is that it is such a southern thing that using it to me with my midwest accent sounds forced and awkward (At least to my ears).
This doesn't seem ambiguous. "All you guys are" seems to be narrowing the focus of the sentence, because otherwise it would be more fluidly spoken as "all of you are", and it sounds unnatural to add to the sentence for no other purpose. (It sounds somewhat unnatural under either interpretation, though). If it had been "you guys are all", perhaps that would be ambiguous, but only with a strong emphasis on the word 'guys', which is not how the phrase is normally spoken. Either way I'd expect the dance teacher to be using hand gestures at the same time to indicate which people they're giving directions to.
Which could mean everyone clear out, or just the fellahs.
I don't get worried about people preferring a term that isn't "you guys", because it's probably an improvement to the language over all, even if it's some friction to change.