Everything is replaced eventually.
That's possible but the question is when?
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.
Who will take over then, I guess Ethereum has a greater chance, it was my bet because I believe in the team behind ETherem and Vitalik was a genius man, he has the passion of what he is doing. If you compare the price there is a big disparity since the adoption of bitcoin is bigger and wider but as you said there will be a change in the future so it makes sense if I will hold some alts to prepare for that time.