Change that happens gradually over several generations is tolerably e. But we have the Internet now. Memes are transmitted worldwide in days, if not hours. If we allow words to be redefined every time someone finds an amusing new way of using them there will be chaos. We won't be complaining that 15 year olds speak differently to 50 year olds, we'll be worrying about 20 year olds not being able to understand teenagers. It will be the story of Babel all over again.
I don't have the book by me now, but he documented a lot words that flipped meaning pretty significantly over less than a half century.