This will depend on the development of bitcoin. If bitcoin remains highly volatile and cannot escape the speculative asset label, it remains subject to manipulation and is frequently dumped whenever there is instability. I don't think banks will add it to their reserves. Because what banks care about more is stability and safety, and that is why they still choose and prioritize gold over any other asset.
Also, this could happen but not anytime soon as central banks are still skeptical and wary of bitcoin. The Fed, ECB, PBOC or BOJ, the world's leading central banks have yet to express interest in bitcoin, let alone add it to their reserves in the near future.
That is true that Bitcoin is a speculative asset and it being volatile won't be considered as a good thing by the banks.
But I think that they will look at how it performed all over these years and that's going to break their standards about a stable asset.
With all of the years of Bitcoin's existence, its tremendous growth and demand could be more than enough for them to accept. Wherever the cash flow is, there go these banks will be interested too.
If they ignore the speculative nature of bitcoin and their own investment principles, and they are attracted to its growth and demand. Maybe they have invested in bitcoin for a long time, no need to wait any longer. But as we've seen and as I said, they're still quite skeptical about it.
Not only banks but also large corporations like Apple, Nvidia and Microsoft are still standing on the sidelines. Even these corporations are in no rush to invest in bitcoin, so I don't think banks will change their minds anytime soon.