Why have some plural pronouns replaced singular pronouns?

I remember when I was very young, my grandpa used to use the words thou and thee and wert and wast, as well as other people in Yorkshire.

When I was at school, these words were used to express a closeness to people, and as I was growing up, they were removing all these words from the old school books, and thou wast became you were. However, there were children who came from close agricultural family backgrounds, whose parents spoke in the old way, and they said things like you wast instead of you were. Eventually, this seems to have become you was, and finally you were, as the schools drove out the old way of speaking.

I think this has also partly happened due to the breakdown of family closeness these days, and the words that had a meaning of this have started disappearing. This also includes words such as shall, ought, hence (in the old imperative sense) and other words that talk of duty, and they are replaced with more relative words that lack strength. People have just become more spiritually and socially insular these days, and the language reflects this.