You have to understand a ponzi scheme.
First, ponzi is only about investment. And by investment, you need to understand “you are asked for value and will be given that value back with interest after a time”. If something is not an investment, then it can’t be a ponzi. Typically, banks produce money, they don’t ask to invest anything. So the term ponzi just does not fit.
Secondly, a ponzi is defined by the interests received being the entry fee of new investers. This means that the underlying activity does not produce value, ie the value of the activity invested in is null. This can happen eg when a genuine activity is actually badly accounted and therefore actually produces a loss of value, so while the original term of ponzi refered to an purposely ill-intentioned scheme, it now loosely refer to the accounting status of an investment - because then it does not need to infer hypothesis on the intentions of the creator(s).
So now you may refer to banks as an investment (because otherwise it can’t be a ponzi). Following the same definition, if you invest in a bank, what is the interest you gain ? Well a bank is paid for selecting which activities profit the more to the economy(=lending money+investing deposits). So they are supposed to have an analysis work. Which in itself is value.
Why ?
First, they are not the same thing at all. Crypto exchanges can’t create currencies. That’s the issue with deflatory currencies : you can’t regulate them. Exchanges are NOT banks.
So while a bankrupt condition for an exchange is the usual one, that is when the losses overweight the gains too much so that it total value becomes negative, the bankrupt condition for a bank is not the same : they go bankrupt in central money, when a customer asks them to transmit money to another bank (eg as a payment) but they don’t have the central money to transmit to that bank. Banks have a lot of mechanisms to prevent that but those can still fail, typically if the changes are so abrupt they can’t abide without huge losses (because if you sell too fast you will plummet the value of what you sell, resulting in an accounting loss based solely on the pressure).
But it is. Holding your deposit is free. They already have all the infrastructure since they are supposed to hold the money they lend you. So banks actually are paid for investing deposits, it is part of their mandate, which is why you can even be paid for this.
Again, many people disagree with that model and I’m not saying this is the universal truth. I’m saying, that’s what bank are supposed to do within that model.