Bitcoin can't be replaceable by any altcoin. BTC is the most successful and promising cryptocurrency, and it has the highest volume of trading among cryptocurrencies since its creation. Ethereum and Ripple do have good potential, but they can't ever replace Bitcoin.
You are a TimeTraveller ?
Say never never!
Indeed. As much as I'm a Bitcoiner first and everything else second, we should never rule out the possibility of anything that is theoretically possible. That would be the mistake a lot of projects made about themselves, and surely the mistake every big company made that they couldn't be replaced.
That said, I think when I die, Bitcoin will still be the Number One.