Paper money would not disappear since its been used for many centuries even there are lots of kinds of currency out there, one would be replace and one currency would exist . You stated that bitcoin would act as secondary, yes it would since theres no possibility that bitcoin could replace paper money since btc was made of codes it a different story and shouldnt be compare.
Well you have a point since technology is upgrading everyday maybe those digital currencies would be adopt by a certain country but i would take time too too replace paper currency. Lets see what would be the potential of digital currency like bitcoin would contribute and developed in the future.