The idea that banks will dwindle if Bitcoin gets adopted is a serious misunderstanding of how banks actually work and the value they provide to people.
We're just not going to arrive in a situation where common people would voluntarily choose to manage their own private keys and crypto wallets, full stop. Can you imagine your parents forgetting the password to their retirement account and going bankrupt? Or your grandma getting tricked into a phishing scam that immediately and irreversibly sends her entire net worth to a scammer?
Even the average Bitcoin user prefers to keep their money in something like Coinbase rather than try to manage their own crypto wallet.
Likewise, gold doesn't simply disappear as an asset and get replaced by Bitcoin. The strange thing about crypto is that the crypto market cap keeps going up as new coins are added to the market. Cryptocurrency is a chance to create new asset classes, which altcoin developers are more than happy to oblige as long as the demand exists. Does this mean more money (value) is being willed into existence? No, in fact there's a lot of evidence that cryptocurrency is heavily fueled by an explosion in credit and leverage. There are literally exchanges that will let you purchase altcoins with up to 100x leverage right now.
The first two are already covered by online banks. People would still need banking even if cryptocurrency somehow got common. Gold would likely not change - there's still trade based around it and currencies are not bound to hold price anymore really.
