To be honest, they should be afraid of their money stored, the existence of quantum computers will make bitcoin the safest assets, because logically banking is easier to break, because the banking system of the security system is not as long as it is on Bitcoin.
The people were not aware of their position, they frightened Bitcoin but had no awareness of the security of their money at the bank.
Although I don't believe in quantum computers because everything is still just rumors, so far. But I think if it can pose a threat to the banking system, bitcoin will be hard pressed to avoid those threats as well.
Banking is a centralized system and upgrading becomes easier when it does not require community consensus, and is protected by laws and governments. So it is very unlikely that quantum computers could threaten them.
To be fair, bitcoin and banking are the safest and most secure systems in the world because they use the same powerful algorithms, and both have their own pros and cons. So if quantum computer could decode those algorithms, both would be at risk, not just one.
Although it is still in the research stage today, it is not impossible to create such things. Similar things have happened in the past. When we wondered whether iron could fly safely in the sky, people in the past also thought the same way before understanding the technology. However, once it was created and succeeded, everyone became believers.
We might be talking about the encryption system used by Bitcoin and also by banks. If banks use the same encryption as Bitcoin, it is likely that it would be resistant to quantum computers or at least difficult to hack. However, if they do not, it would not be difficult to hack, even if protected by regulations.
Precisely because it is centralized, the money in banking is not in a secure state where they have control over the money you own; they can freeze or manage your money as they please.