Paper money will stay forever.
Why? Because its easier to make than coins.
The paper money will exist forever because no matter how many bitcoin we have keep in our wallet in the end we will convert it into cash. Many people around the globe dont know how to use bitcoin and they dont know how it works especially the uneducated or the poor who not even know how to use computer for digital transactions.
Paper money has its own place in the world economy system and I think it is really absurd to compare bitcoins and paper money or to think that one is going to replace another forever. This is really not possible because we need both systems of currencies in to meet the demands of present era.