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 Since the world started, many currencies have been created and have been replaced by another one.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
I haven't been paying attention to ZCash, but I've been following Ethereum for a few years now, and I think that there are many in the community that are underestimating its potential. They're are either "diehard" bitcoin enthusiasts, they just aren't paying attention, or maybe they just don't recognize its potential....I don't know, but I don't think bitcoin will ever become obsolete as long as it has these type of supporters to secure the network. Bitcoin and Ethereum are two different type of creatures and there is no reason why they cannot co-exist together. I don't think the Bitcoin network is threatened by the Ethereum network.