Bitcoin was designed as an investment vehicle by its choice of capped supply.
Use as a currency would have vastly benefitted from a fixed block subsidy, which also would have avoided much of its current wealth concentration [1].
Use as currency is possible with Layer 2 systems like Lightning. The issues I believe are both technological, political and philosophical: apart from the ease of use (or lack thereof), people prefer the currency they need to pay taxes with, and will always prefer the depreciating one, until it stops having any useful value. See the arguments put forth by Saifedean Ammous in his book.
For me the main ledger is equivalent to inter-bank settlement payments. It is not for people to use directly, it is to settle the large amount between "market makers", so $10+ transaction fees are not a problem when the amounts transferred would be in the millions. These settlements are few and far between, they do not require immediate execution. In Bitcoin parlance, these market makers would be Lightning nodes, which I imagine in the long run would not be operated by your Average Joe.
Bitcoin (BTC) worked perfectly as currency in the beginning, until Blockstream took over the development team, and they decided to keep the temporary anti-spam 1MB block limit, making 0-confirmation transactions (instant payments) very risky by implementing return-by-fee (which make transactions reversible before getting confirmed), etc.
Fixed block subsidy would have a risk of supply outpacing the demand. Then the price would drop structurally which could cause "run on bank" and wholesale abandonment.
Decreasing amount of bitcoin supply means that eventually the supply would fall below demand, which ensures rising price.
Every 4 years when supply and the demand reach equilibrium, the supply is halved which triggers new price rise and renewed interest. The entire crypto ecosystem follows.
Bitcoin halvings are THE reason crypto is a thing. Not just a curiosity for some techies.
We have maybe two cycles left ahead of us. Then crypto will become just digital gold randomly fluctuating with interest from the asset holders.
For me the main ledger is equivalent to inter-bank settlement payments. It is not for people to use directly, it is to settle the large amount between "market makers", so $10+ transaction fees are not a problem when the amounts transferred would be in the millions. These settlements are few and far between, they do not require immediate execution. In Bitcoin parlance, these market makers would be Lightning nodes, which I imagine in the long run would not be operated by your Average Joe.