Re: an informal GM with K

English, like most languages, borrows from other cultures. That's normal. But English, because of the diversity of its speakers, likes to grow fast in a gazillion directions at once. This would be defined as a fad. But, like most fads, they die out due to lack of use or boredom. Thee and Thou died out, probably, out of boredom. That's my guess. English does keep some formalities here and there. But it is such a fast mutating language that things that aren't useful, don't last for long.
Thanks still has its formal Thank You but Hello is slowly going by the wayside. Hi is starting to be acceptable as formal. Give Hi another 10 years and it will be on formal letters by then. My Guess.