While quantum computing might not be a threat to bitcoin directly, yet it is a threat to the security of funds when you reuse addresses. This is because when you use an address, your public key is exposes and with quantum computing, it can be possible to compute your private keys from your exposed public key.
It is better you don't reuse addresses, You can mitigate this risk by moving your funds to a new unused address each time you use an address so that your coins are safe in the new unused address.
Yeah this is also what I understand the threat to be.
QC can break encryption from public key to get the private key. So QC is only a threat for when you reuse an address as the public key is only exposed when you send a transaction. Though it also means the ~2 million bitcoin sitting in addresses from back in the real early days of bitcoin (when bitcoin was sent directly to public keys rather than public addresses) are just sitting around ripe for the picking for when QC gets mature enough to break the encryption. So that is the main threat, the ~2 million btc that WILL eventually get stolen by QC at some point in the future. Of course some of that btc are still controlled by people and they can move it to new addresses, but the majority of those coins are likely lost coins, and so would eventually be stolen in a QC world.
I did see recently there is some proposal about eventually essentially blacklisting those QC-susceptible addresses basically, or I think maybe the proposal was actually to have everyone move to an even newer address type and not allowing any current address types in the future, i don't remember what the exact proposal was, but that is what I proposed on here maybe a couple years ago - that unless some action is taken there will be ~2 million bitcoin that get stolen at some point in the future from QC and so at the very least that original pay to public key transactions should be blacklisted, thus preventing the threat of QC stealing those ~2 million lost bitcoin from 2009/2010 time period.