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.
I was making a broad point about the role that gold plays in our society today versus, say, 100 years ago. I understand that my numbers are just guesses, but the exact number wasn't the point: gold, as an "investment technology" has becoming a very tiny portion of the overall picture even though it was once dominant. That could be the case for Bitcoin and digital currencies in the future as well.
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.
Bitcoin itself is decentralized--nobody disputes that. But the way most consumers use it is
centralized, so it doesn't matter if Bitcoin itself is decentralized or not.
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.
If you use an L2, you are betting on the integrity of the L2--along with Bitcoin itself. Whatever disadvantages the L2 has, your investment overall has. Hence an L2, regardless of the L1 behind it, is absolutely a
centralized investment (and yep, investors don't care, because they don't care about decentralization).
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.
The market is going the other direction, and has been for years now. The ETFs were the nail in the coffin for decentralization: it shows that investors simply don't care about it, and they just want to invest in a meme that goes up in price.
I have no idea what "events" you could be talking about, but I can't imagine something that is going to make people dump all of their ETH and other non-decentralized cryptos, dump their holdings in brokers and apps like PayPal and Coinbase, dump their ETFs in favor of self-custodial pure Bitcoin, and dump L2s. Why any consumer, let alone millions of them, want to do that?