Gold, silver or even shells can be a good medium of local exchange in the cases where bitcoin becomes inconvenient.
Alternatively, a bitcoin backed note could suffice as long as the issuer is trustworthy. This is possible in local contexts, but not at scale in the global context.
I like this, as it gets at the fundamental nature of what money really is. It's a medium of exchange for goods and services, so anything that suffices to make exchange possible will do. There's no real reason we need to use the same currency in different domains, as long as everyone involved agrees on the value being exchanged.
