There are already companies that keep bitcoin as reserves; in time, banks will also have to adopt this since they can't beat the system and will have to join it.
Check out the list of companies using Bitcoin as reserves; it might be outdated somehow, as I believe companies like Tasla might have sold out some of their Bitcoin reserves.
Surprisingly, some of the companies that on the list is already file bankruptcy like Voyager while Tesla recently announced that they are already sold their Bitcoin holdings.
I think the OP pertain the reserve currency of country that holds by the central bank and not as a reserved balance of a corporate which the list all about.