> Bitcoin was designed as an investment vehicle

Please read the whitepaper [0], at least its title

[0] https://www.bitcoin.com/bitcoin.pdf

I guess it was lucky that it was designed quite badly for the stated purpose and it was actually designed as investment vessel.

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.

It's the miners and users who ultimately decided if the 1MB block limit should be kept.

Then the miners are clearly smarter than the users

Why? They decided the same. Both miners and users favor the fork where the 1MB block limit is kept.

Bitcoin Cash and Bitcoin Gold are used and mined very little.

Miners mine whatever is most profitable for them, regardless of how good or bad is that cryptocurrency, and they switch between BTC, BCH or any other SHA256 (not Bitcoin Gold, which uses a different hash function) even many times in a given day.

Yes. And users use what's most profitable for them. Compare usage volumes of btc, bch and btg.

Network effect

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.

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.