I agree with you that nothing lasts forever, there will come a time when a better currency than bitcoin will come along and can compete with bitcoin. But I don't think that will happen anytime soon and if bitcoin does have competitors it certainly won't be the current altcoins.
Nothing is absolutely perfect, including bitcoin. Like gold, it also has the disadvantage of being bulky and making it difficult to store and transport. But why is it still considered the best asset and no one can replace it?
Gold is perhaps 0.1% of the world's reserves in terms of a store of value. It used to be something like 50%.
Nothing is going to happen to Bitcoin, it's just that alternatives will become much bigger than it.
Even if more altcoins are created with new and better technology, but if they are not decentralized and do not guarantee the security of the blockchain, they will never be able to surpass bitcoin, let alone replace it.
Consumer investors have demonstrated, based on their actions, that they don't care about decentralization. Today most Bitcoin is held using a person or company's known identity, using a broker or an app. And there are thousands of L2 "bandaids" on top of Bitcoin that basically eliminate decentralization--and lots of people are using these too. And then there's the ETF. And almost every crypto
besides Bitcoin--which makes up about 45% of the total market today--is based on the defacto centralized PoS model or something similar.
Consumers simply don't care about "decentralization", and I suspect 95% of investors in Bitcoin don't even have the first clue about what that word even means. The market is clearly evolving away from this notion and it has been for years now.