I think bitcoin will never be replaced by other digital currency. bitcoin a digital currency is a first created, bitcoin has historical value as well as high value. there will be no altcoin that could change the position bitcoin in the main option.
Everything will change even if it is Bitcoin. Since the world started, many currencies have been created and have been replaced by another one. Bitcoin is the first digital currency so that it is being used by many people. However, in the future, many new cryptos will be made which will be more effective and more secure. Right now, Bitcoin have many dangerous competition such as etherium, zcash
lol what is that zcash? even ethereum that take people attention when they are officially partnering with microsoft as a service on azure, they are mean nothing to the giant cryptocurrency bitcoin , it's possible indeed to have a bigger and better cryptocurrency rather than bitcoin , but there is no bigger name than bitcoin !