It will be forever, bitcoin has been the first cryptocurrency, and it would not be replaced, maybe there are better altcoins with more advanced technologies, but it doesnt make them better, because those people who have never invested in bitcoin, will invest in btc at first, not in other currencies, so it is more famous.
But no, it would never be replaced..
Some altcoin may be better but that doesn't make it better. See a problem with that statement? I guess you're saying bitcoin is a better investment despite the altcoin being better, technically.
But won't the better coins eventually win?