I see what you mean. After all, 1 Bitcoin will always be equal to 1 Bitcoin no matter what. It's all about utility instead of speculating crypto's price/value in terms of Fiat. But since many people view Bitcoin to be an investment tool, it doesn't hurt good to speculate what its price will be in the future. Gold and other precious metals are also valued in terms of Fiat. Some are much more expensive than others, but they're also speculative like Bitcoin is today. When you have 1 oz. of Gold, you're confident that it'll be that way for the foreseeable future. What changes is its price on the market in terms of Fiat. In these times of need, Gold has risen a bit in terms of USD, while Bitcoin has recovered to its pre-COVID prices on the market (sort of).
If Bitcoin gains massive mainstream adoption due to the collapse of the traditional monetary system, 1 satoshi could be well worth 1 cent in terms of USD in the future. There's no denying that purchasing power decreases on a currency that's experiencing constant inflation. This effect would turn out to be positive for Bitcoin "hodlers" as they gain greater purchasing power due to decaying Fiat. In other words, the more banks debase Fiat, the better it'll be for the long-term price of Bitcoin on the market.
But first thing's first. People would need to use the Lightning Network more thoroughly, in order to reduce the burden on the main Bitcoin blockchain. When prices reach exorbitant levels, it'll be possible to use units of a Bitcoin (in a form of satoshis) for the decentralized economy. Otherwise, paying cents in a form of 1 satoshi on the main Blockchain network, will not be feasible.
even though many experts say that Bitcoin is a bubble, but I think it still has the power to be a useful investment
For a satoshi to be worth $0.10, Bitcoin needs to reach a price of approximately $10 million USD. I think that's insane since Bitcoin's supply is not very limited. For 21 million coins that will ever be mined, I think $1 million USD is a more realistic target. That means that 1 satoshi will get to be worth $0.01 cent (USD) in the future. No more than that. If it does get past $0.01 USD per satoshi, then Bitcoin would've been overvalued. But who knows? In this uncertain world of crypto, anything could happen. If you truly believe in Bitcoin, you would "hodl" it for the foreseeable future. Spending a little amount of coins, greatly contributes to the crypto economy. Just my opinion