The economy size of the country will be determined by the money available in the country physically and not electronically. So paper money will never be worthless even after 20 years.
20 years is a really small period of time, we should think about higher then that, and for sure there are chances of digital money taking over paper money in future, because every thing has to be updated once after it gets too old and now the technology has reached to the sky and i'm sure this revolution will take place too.
Paper money will be there for a long time because we haven't found a way to make internet connectivity and electricity to be available
in every part of the world. There are millions of people who are not connected to internet and are not aware of it. For them, paper currency
is the only option.