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.
Not disputing that there is a chance bitcoin gets replaced, but I very much doubt it will be ethereum. Just about all the problems people see with bitcoin, like governance, scaleability etcetera, ethereum has as well but with more complexity. I don't understand why people don't seem to realize that Ethereum has not actually solved anything yet (and no, a centralized dictator enforcing his will is not a better solution). Ethereum does not scale better than bitcoin, I don't understand why people try to push that idea, (just look at the size of the blockchain for a full node). And their choice to make the chain muteable was a horrific move and clearly shows the centralization problem in ethereum, why even bother with the hassle of a distributed chain then? Listening to a number of developers and their frustration with getting anything remotely serious done with it, I'm more and more getting the impression that ethereum will never be able to live up to the hype and has some serious limitations in the way it was designed that will not be resolved by any fork or update.
Last but not least a number of those devs complaining about ethereum seem to be hyping up for a new blockchain project that apparently does not suffer the same problems as ethereum. If what they tell me is even partially correct, ethereum will be first in line to be replaced within the next few years.