For example, you can't lend Bitcoin in a decentralized way, so a bank would be needed for that.
I think I'm missing something here... How could any of the traditional banks, help you in lending your
BTCitcoin in a decentralized way?
My point was, if you can't lend or take a loan in decentralized way, you would need to do it in centralized way, so banks are needed for that. Of course you could make a private contract without intermediaries, but it would be too risky, so it's better to put your coins in a bank if you want to earn interest.