You

Change is a part of language.  Of all the ways to justify a pedantic claim that one's pet usage is "correct" and Kids These Days are borderline illiterate, the appeal to history is one of the weakest.  OK, so originally decimate (or rather, decimare) referred to a randomly selected tenth of an insubordinate legion.  That meaning hasn't been current in English for decades or centuries, if indeed it ever was.  In today's English, decimate means "destroy almost totally", because that's how people use and understand the word.

(Not that I don't inwardly cringe when I see the growing use of the possessive marker 's for the plural marker, as for example "Employee's only")

One change that appears to be gaining acceptance is the use of they as a gender-neutral substitute for he or she, handily filling a gap that seems to have grown more noticeable over time.  It has the advantage of being a real word, as opposed to any of several newly-minted words that have been proposed for the purpose, so people already know how to pronounce it and what verb form to use with it.  Of course, everyone knows that they is actually plural, and so using it as a singular is incorrect and thus liable to lead upcoming generations horribly astray if not destroy their verbal capacity entirely.

But of course, everyone is forgetting about you.

Just like they, you is syntactically plural.  In particular it takes the plural form of its verb, as in you are.  Nonetheless, it is used for both singular and plural.

English is unusual in this respect, at least as far as European languages go.  Most European languages use separate forms for singular and plural in the second person, as indeed English used to.  The singular form is almost always tu (as in the Romance languages) or some variant (e.g., German du).  There is more variation in the plural form, though in the Romance languages it's consistently a derivative of Latin vos.

So why would English not distinguish, but instead use the plural form for both singular and plural? Well ...

European languages don't just distinguish singular you from plural.  They generally also distinguish familiar from formal.  For example, in Dutch, a shopkeeper or bank teller will generally address a new customer as U, but two friends or family members will address each other as jij (pronounced "yiy" to rhyme with "sigh", or "yuh", depending on whether it's stressed).  There are two main patterns for this:

1. The formal comes from the third person, as with German Sie or Spanish Usted (from Vuestra merced, literally "your mercy", more loosely "your grace").
2. The formal comes from the plural, as with French vous.

European languages also tend to distinguish case, as English still does with I vs. me, he vs. him and she vs. her.  English has lost almost all of its other case distinctions and seems intent on losing the rest.  The who vs. whom distinction is essentially gone, and even he/she vs. him/her only seems to matter in simpler contexts.  In my own experience, most people will say either me and him, if they think no one's looking, or he and I if they think they need to use "proper grammar", regardless of the actual case involved.

What does all this have to do with you?

English used to use a perfectly ordinary European system: singular thou (cognate with tu) and plural ye, with the plural for the formal.  In the accusative case (direct object of a sentence), the forms are thee and you.  So

• Shall I compare thee to a summer's day?
• Thou art more lovely and more temperate.
• (I had to go back to Chaucer for clear examples of you as opposed to ye):  But first I pray you, of your courtesy/That ye narrate it not my villainy

I don't know the actual order of the first two events, but over time

• The ye/you case distinction was lost in favor of you
• The you form became the (singular and plural) formal as well as the plural familiar
• The familiar/formal distinction was lost, again in favor of you (to the extent that to modern ears the familiar thou tends to sound "formal")

So, behind that simple word you, one-size-fits-all-numbers-and-cases, lurks an elaborate structure of case, number and familiar/formal distinctions, most of which is now long forgotten.

And if only it were that simple.  There are plenty of wrinkles in the basic pattern of "tu, some plural form, either plural or third person for formal (both singular and plural)":

• German uses third-person (Sie) while its close cousin English used the plural (you)
• Dutch (about as close as you can get to both German and English) uses different forms (jij and U) both cognate with the English plural you (and German accusative plural euch), for the singular familiar and formal respectively.
• Spanish uses a third-person formal, but the Vuestra in Vuestra merced implies that it had previously used the plural vos.
• Spanish distinguishes between formal singular and plural (Usted and Ustedes -- Vuestras mercedes), while most European languages only distinguish singular and plural in the familiar form
• French uses the plural vous for the formal -- actually, French is relatively straightforward in that respect.
• Italian has both (Lei, a third-person form, is more widely used, but some dialects retain the older voi).
• English has several unofficial plural forms (y'all, you guys, youse, you-uns etc.), which leave the once-plural you as a purely singular form