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.
Altcoins will only function as a speculative investment option for greedy people. Serious investors always stick to Bitcoin as it has the best possible chances of surviving.
Of course there are people thinking that altcoins may actually replace Bitcoin eventually, but they don't know what they are talking about.
Ethereum was pointed out by a group of people to be the Bitcoin killer, but the only thing it has been successful in, is making the people behind Ethereum multi millionaires.