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..
Nothing is going to last forever
Ans still more so in the world of crypto. More specifically, Bitcoin without doubt has the first mover advantage, but its real advantage and edge over other coins is that it is capable of bringing in more profits over longer terms than the rest of the pack. This is the thing that keeps it at the top. If it loses this capability due to some "natural causes" (like those Hue Hefner died of yesterday) or because of some external influence or impact (say, China explicit ban), it will lose its top place since people will instantly switch to something more profitable