The reason why "you" completely supplanted "thou" in contemporary speech is because nobody wanted to appear disrespectful of anyone.
Are you sure this is the reason?? In Russia actually I say "you" to acquaintances sometimes by accident, and they can be confused. Sometimes they ask "Why do you say you to me? We are friends!"
They get upset as if I am now putting distance between us as strangers.
Perhaps in modern, more independent and prosperous America, friends are often not as close in relations as centuries before, when they had to rely on eachother, and this explains it? How else to explain that the Quaker method of friendly egalitarian informality (which I like) lost out to the more formal way of expression?
This linguistic tendency has nothing to do with modern America. It developed many centuries ago in England.
Even in Old English, "Thou" could be used in a pejorative sense by placing it in apposition to a noun in the vocative. The Oxford English Dictionary (OED)
has the first such attestation in King Ælfred's Old English translation of Boethius' De consolatione philosophiae
. So, things start in that direction in the year 888 A.D.
There are other such examples throughout the centuries, especially after the French influence on English, until 1530 A.D., when the OED records this great line, which turns thou into a verb: "Avaunt, caitiff, dost thou thou me! I am come of good kin I tell thee!" So, by the 1530s, calling someone "thou," or "thouing" them, could be seen as disparaging their lineage.
That's why Sir Edward Coke in 1603 could mock Sir Walter Raleigh in court by saying: "All that Lord Cobham did was by thy instigation, thou viper; for I thou thee, thou Traitor!"
After the KJV, one doesn't find many uses of "thou" in the OED that aren't from poetry or in some work that is imitating the speech of ill bred/uneducated folk. The Quakers, of course, had to make an intentional practice of saying "thou." In other words, even in the 1650s, it was not normal speech to do so.