I don’t see your point here OP.
Classic has been up for 2 years and all the content is over now. So its pretty much expected that most people will either quit the game or look for something different, and that is naturally TBC since we know Blizzard doesn’t have the capacity to make great games anymore.
So there is nothing wrong with going into TBC, and its also very good that they give you the option to stay in vanilla forever if that is what you enjoy.
On private servers after 1-2 years they would reset all progress and start fresh for the same reason.