Non-payers are the whole reason banks go under.

If a bank has $1,000,000 in deposits, they can lend out $10,000,000. If the bank does not get any of the $10,000,000 back, they can no longer cover withdrawals if someone were to make one (this is the liquidity crises). As soon as a bank can no longer cover withdrawals they are insolvent.

No different than what happened in the sub-prime crises, only what I just mentioned would happen on purpose. And unlike real estate, the crypto could not be recovered..

As far as the credit card defaults, collection agencies would also start to bankrupt if there is no money to recover.. You can't collect crypto debt... So they would have to stop buying it.

The key is that it would have to be a significant percentage of the public doing this in order for it to succeed...