They can't, crypto isn't a stable asset it's highly volatility and far different from money, it's going to be hard to see crypto and medium of exchange really because the volatile part doesn't guarantee anything
Inestability is not a real problem, banks have worked with mortgage backed securities in 2008 and everyone knew that these were not really stable not worth much for that matter, yet still they sold them around until the last fool standing saw them drop to zero. Anyway, crypto can be used as any currency with fluctuating value - that is all of them - even banks will use turkish liras and the like when their clients demand so, so crypto is not different nor should be.