This is linguistics by way of statistics. Statistics graduate student Joshua Katz made heatmaps based on dialectal differences in American English found in a survey conducted by Bert Vaux, a linguist at the University of Cambridge. I wanted to highlight this slide because it so clearly demonstrates the cyclical nature of language change.
This image shows the second person plural pronoun as used in contemporary US English. As you can see—and probably already know from personal experience—very few actually use “you”, the same word used for the second person singular. This is a useful grammatical distinction that English used to have, then lost, and has now developed again.
In Old English and early Middle English, the 2nd person singular was thou, oblique form thee. This form was originally neither formal nor informal, used to address people of any rank. The 2nd person plural was ye, oblique form you. So how did all these four forms collapse into just one, and the one that used to be the objective plural? After the Norman conquest of England, French started exerting a strong influence on English. Before this, the dominant influence had been Old Norse—the pronoun they is of Norse origin, supplanting original English forms that were too easily confused with singular forms.
French had the T-V distinction, a cross-linguistically common distinction between formal and informal pronouns of address. Under this influence, the singular forms thou/thee came to be regarded as informal, although they, too, have come full circle and are now regarded as extremely/comically formal because they sound so archaic. The polite way to address a singular person now became the plural form, ye/you. These changes also coincide with a general collapse of the complex morphology of English, including its case system. Eventually, thou/thee fell out of favor, and the distinction between ye and you disappeared, leaving only you.
The T-V distinction is common across many languages. Seeing as I just had a Russian exam today, I can tell you that the polite way to address a solitary stranger in Russia is вы (vy, 2nd person plural), which is also how you would address a group of strangers even if you wanted to insult them. But the distinction between singular and plural is also very common, because of its usefulness. And so we’re seeing this distinction creep back into English again, with terms like you guys or y’all. But unlike in the olden days, this time around it’s actually the plural form (you guys, y’all, youse) and not the singular (you) that’s regarded as informal.
This theme of useful distinctions collapsing and then being reinvented is a common one throughout the history of human language. It’s especially silly to talk about linguistic purity or the degradation of language when you have this in mind. Language changes all the time, everyone agrees, but some—often smart, educated people who lack linguistic training—are especially vehement about its being a bad thing, that change is almost always bad or degrading, that language is devolving. But as it turns out, linguistic change is more like a circle, spinning round and round, losing complexity here, gaining it there, then reversing.
Proto-Indo-European, the unrecorded language spoken more than five thousand years ago that spawned modern-day languages as different as Farsi, Russian and English, had a highly complex morphology with lots of cases, three numbers (singular, dual and plural) and so on. Old English had already lost some of this complexity, and Early Modern English had an even simpler morphology. But now modern American English has reintroduced at least some of that complexity to the pronoun system.