That's easy. As with all things, when something better comes along bitcoin will be replaced.
The technology, computer science and programming world doesn't sit still in a vacuum. It won't take as long as people believe.
Bitcoin cant be replaced by other coin. we all know that bitcoin has some major problems to be fixed but the project is the main root of all the cryptos. bitcoin will always be the known cryptocurrency. but for some instances if the bitcoin is replaced i bet it would be some coin that have potential with their own codes that hasnt copied on the bitcoin's.
No one can say exactly what will happen to bitcoin in a few years. To date, he is the strongest, but after a while he may die due to the fact that they will create a more perfect coin and users will stop making deals with bitcoin.