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Check out the list of companies using Bitcoin as reserves; it might be outdated somehow, as I believe companies like Tasla might have sold out some of their Bitcoin reserves.
It is not at all necessary for banks to join and use cryptocurrency if they cannot beat it. The banking system is a part of state power, and states always use such leverage as regulation for undesirable processes. By regulating the circulation of cryptocurrencies, states can practically achieve any effect. Banks use everything from cryptocurrency that is beneficial to them to increase the efficiency of their fiat system, including they, like the state as a whole, use blockchain technology. But they will not use bitcoin or another decentralized cryptocurrency as a reserve currency, as this is not acceptable to strengthen their fiat. Commercial structures, individuals can and do do this, but this will not affect the state banking system in any way.