I dont think it could ever happen in future that bitcoin being replaced by any other crypto-currency.As of now we see any other crypto-currency has no match with the bitcoin till now.Ethereum promised alot in the start and looking like a good compatitor of bitcoin but it is now too far away from bitcoin so i think bitcoin will rule the crypto-currency world.
There is always this possibility that someone will take the seat of bitcoin as the number one cryptocurrency but no one can replace bitcoin in terms of what it has done to the financial and investment world. Pioneer will always get the credit even if it become obsolete. But the demise of bitcoin depends on the decision of those who have the authority over the changes and implementations of bitcoin.
ZCash could be the one to take the seat of the bitcoin or become a second coin as it offers the anonymity in transaction.