At least most banks across the world have liquidity requirements, and most have some insurance for limited amounts, for example if you want to open your own bank in Cayman's you need to have 30-50 million in liquid assets such bonds and gold deposits, it is similar for US banks, if recall correctly.
You might be able to transfer bitcoins from valet to valet, but they will be effectively useless, if banks stop releasing cash, cash will be the most valuable commodity, not digital bits. Furthermore, during the 90's (the really crazy times) in Russia many were doing bond trading just fine without any issues and without internet, this not some story, it happens so I met a few survivors from that time.
On another hand, the only useful thing I find for bitcoin that is "legal" is contributing to sci-hub projects, nothing else, for this reason and other reasons such as drugs and money laundering, bitcoin is here to stay.
