Yet
This concept that language is set in stone is entirely ridiculous. English has been continually changing for as long as it has been in the British Isles. Each change would have been considered a 'devolution' by many. The truth is that it's simplification.
You know "aks", that misspoken "ask" that people consider a sign of stupidity? Chaucer spelled it "aks". This was a time when English would differ over small geographic spaces because the written word was only a representation of the spoken word, and the spoken word always differs depending on accent, culture etc. If an English person wrote on, for examples sake, a Facebook update: "Going t'pub for a pint", and someone else wrote "dis pint is warm", what's the difference? One is quaint and seen as cute and traditional, the other is a sign of English's direction, so it's new and scary.
In terms of personal pronouns of the second person we have, since Old English, moved from:
þū (nominative singular)
ġē (nominative plural)
þē (accusative singular formal)
þeċ (accusative singular informal)
ēow (accusative plural formal)
ēowiċ (accusative plural informal)
þē (dative singular)
ēow (dative plural)
þīn (genitive singular)
ēower (genitive plural)
In the Middle Ages 'you' and 'your' replaced all formal singulars and formal plurals, while the informal singulars and the informal nominative plural were replaced with 'thou', 'thee', 'thine' and 'ye'.
Today, it's 'you' and 'your' in each case. What a devolution, right?
). 
