I personally can see a world where bitcoin's marketcap and activity is eclipsed by a future altcoin. Ethereum is getting very close. But in regards to "replacing" bitcoin, nothing will replace bitcoin. Bitcoin will always be the first cryptocurrency, it is the most decentralized coin and it is literally the underpinning of the entire crypto economy. Nearly all coins are quoted against BTC. This won't change in the same way the USD isn't going to be usurped by the yuan for an example.