Bitcoin is the original cryptocurrency. That alone gives it an advantage over other cryptocurrencies. Even if there are many other cryptocurrencies which have started following the footsteps of bitcoin, none of them are as successful as bitcoin was when it was released in 2009. The reason for this is the fact that Bitcoin is safe and secure and people trust it enough to use it just like they use dollars, euros or pounds.
I think what gives Bitcoin the advantage over other crypto currencies is that it's the first introduced blockchain, so it took the first step towards blockchain technology adoption. Thus, it has the respect of all crypto holders whether they like it or prefer another crypto currency. Another reason, in my opinion, is the anonymity of its creator, which is considered a good point for legal reasons.
I personally like Bitcoin and prefer it over the others for now, but still think that one day another crypto currency will replace it, maybe Ehtereum, which is developing very well, maybe another new one. However, Bitcoin will always be remembered as the first viable blockchain.
I don't think ETH or other altcoins will replace bitcoin in the future. I agree that there is still a chance that bitcoin will be replaced by another coin, but it has to be a really better coin than bitcoin and not the junk coins of today. While bitcoin brings a lot of utility and use cases to us and is entirely decentralized and not under the control of anyone. Altcoins are just projects created for the purpose of pumping and dumping, scamming people with promises of high profits, they don't have any purpose like what bitcoin has provided us. Therefore, it's almost impossible for altcoins to replace bitcoin if altcoins continue with what they're doing.