Banks will not enter these deals, so what will happen if I locked my currencies in the bank and the price continues to drop, then the bank will lose.
Building on the assumption that the price will continue to rise is unrealistic.
Since we are talking about new technology, we will not go back to using traditional models such as banks. Rather, there may be portals that decentralize currencies and use smart contracts between different blockchains. Then the need for cash may be less than now.
What the price of bitcoin does doesn't really matter in this regard. Why? Because bank is getting 50k but also getting stablecoins from Joe as payment, if he doesn't pay up then they will not only take his coins but also will be able to put a seizure on him as well, basically take his things, literally anything they find at his house suddenly becomes banks.
So that is why it is still valid, hell even without the bitcoins it is valid, because that is what they do, if he has 50k in bitcoins and can show that as proof of income or value, he could get a loan without giving that bitcoin. I can take a loan, I show nothing of worth now, and the only difference is I am doing it on fiat instead of crypto but as soon as I get that loan I can turn it to crypto if I want to. All of these are basically the reason why we can have this type of deal in the future.