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.
You are quite wrong about a few things here. The first is that you think its even possible to calculate the total value of all assets, and then gold (or BTC's) share of that total value. It is more possible to do though is you define specific financial assets to compare against (eg total value of bonds, cash, equities, property, collectables, etc, but even then its not easy as there are many other global assets not in any of these categories.
You are also mixing up various concepts related to decentralization. Some aspects of BTC are highly decentralized, and are increasingly becoming so. Others are indeed becoming more centralized. We will see the tides of centralization and decentralization of aspects related to BTC grow and shrink over time for numerous reasons. Yes, we will see centralized L2s on BTC. That is fine, provided BTC itself, the ultimate store of truth, remains rock solid.
As for consumers not caring about decentralization, they most definitely do (even though they may not know it yet), and will increasingly do more so as events occur which make them understand the importance of BTC's unique characteristics.
There is really only one increasingly dominant market for a global decentralized digital store of wealth, and that is BTC, with the vast majority of "alts" simply not even in the same category.