Decreasing is the right word but not totally erased. There are many things that cannot be done by electronics. What if everything is down, an electricity outage then what are we going to do to buy our stuff or our needs. That is why it is still needed.
Yeah the usage of bitcoin will increase in future but it doesn't indicates that paper money will simply disappear in future, paper money is easy to use anywhere to buy anything.
That is why paper money lived for many years because of it convenience to every people. If it had not worked for them maybe they will change it but it did so it will be hard to take that away.
People will prefer to use bitcoin more in future as we will also find many physical shops where we can spend our coins directly, but I think the presence of paper money will always be felt.