I don't think this is a realistic scenario at all.

There isn't really much of a probability that bitcoin will ever replace fiat completely, and there will probably always be some form of centralisation in the currency supply because of the fact that centralised, and government issued cryptos are going to be issued in the future. It's hard seeing that decentralised cryptocurrencies such as BTC being the only currencies used globally, even though I do expect it to go mainstream.

Besides, it's impossible to predict whether or not there will be financial intermediaries that act in the same way as these investment banks did in the crisis with mortgages. Just because the new currency may be decentralised doesn't mean there couldn't be loose lending, or fractional reserve off-chain.