Thou was the singular you. But some lazy/ignorant people started using you as the singular. Other people were too lazy to correct the mistake. So we lost the meaning of you and had to create kludges to work around the loss of meaning: y'all, yous and the like.
What false nostalgia. Many of these older, seemingly more "elegant" features of language were themselves kludges once upon a time. Once we forget the origins of words, we take them for granted, whereas words like "y'all", the origins of which we remember and can locate in a familiar place/time/culture, seem mundane or even crude. If a few centuries were to pass and "ya" and "yal" became the two second person pronouns in English, it would inconvenience hardly anyone.
Eventually "Thou" became the familiar form, used with those you were close to--such as family. "You", originally the plural, was then the more formal form. Once we stopped using the familiar form, we were left with the ambiguity that singular and plural second person pronouns were the same.
But there's always context to consider. How about asking someone on the street, "How are you?"--and they start telling you how their health is. It's a formal idiom, and doesn't mean exactly what the words say.
If I really want to know how you are doing, I should repeat the question, or expand on it.
It's a matter of ignorance to think there is. This is what we get when we encourage the idea that words mean whatever anyone thinks they mean. This is why people say decimate when they mean annihilate or obliterate.
I agree with everything you said in this post, except a particular point here. As I understand it, the common use of the word "decimate" does not refer to the strict definition (reduce by one-tenth), but the general context of the word as it was first used (punishing a conquered enemy by killing one-tenth of their men, presumably a pretty severe punishment). . . .
Actually, "decimation" was even more horrific. It was a punishment applied to Roman troops that revolted (or committed a similarly severe crime): the troops themselves had to kill one of each ten men. As they lived in tents of ten, that means one of your buddies was chosen at random, and you were given clubs to beat him to death. Truly horrific.
But I do agree about other words used, often in error. "Unique" is one common example--"peculiar" or "unusual" is usually a better fit. Why take a word that is, well, unique, and make it into a synonym of several other words?
Another funny example is "literally". As in, "They were literally exploding with joy."
Having become a Wizard on n.p. 2183, the Yellow Piggy retroactively appointed his honorable self a Temporal Wizardly Piggy on n.p.1488, not to be effective until n.p. 2183, thereby avoiding a partial temporal paradox. Since he couldn't afford two philosophical PhDs to rule on the title.