Banks use tools as a store of value by holding some assets in reserves, for various economic benefits. Although, adopting Bitcoin in such capacity, would have it's pros and cons. Obviously like controlling inflationary trends, and even help stabilize the volatility of Bitcoin to its users, and conversely such volatility could also alter the stability of a banks statement of financial position. All these could add up to a more stable global economy. Surely banks have started to think of such prospect
If the banks can hold Gold for long time and they didn't collapse, they can hold Bitcoin as reserve and nothing is going to happen to them. Do you know many times banks print fiat when economy isn't suitable like they predicted, the US do it once in a blue moon, and for what? It's spending. It's this printing of money that saves their ass and think Bank is a superiority, better than any other financial institutions that exist other their closed monetary policies.
If Banks so wish to buy and hold as reserved, they can do it and there wouldn't be any consequences but they wouldn't do it. You know? It's because it will be like they are given a strengthen hand to Bitcoin, it's like endorsing it as an investment like s&p500 list where potential money will come. Imagine if all the top US companies start selling their shares and buy Bitcoin, alot of people at top 100 wealthiest people will become nothing, they will go broke because their shares value is going to drop.