BTC is not something that you can just replace that easily. It is still growing and getting better over time so although nobody knows about the future i can tell that in the future bitcoin will exist if there is anything like bitcoin it will only become second to bitcoin and never get on top of it.
Absolutely with the current price of BTC i'm in doubt another currency would replace it on top. It is already tested so many times that what ever negative news against BTC though it temporarily its price fell down it always show resiliency and continue to break its own record high.