Can someone explain why quantum computing is not a threat?

For the record, I’m a big believer in bitcoin and plan to hold for the long term. However, I do think quantum computing poses a significant risk. I hear people discuss that we will simply switch to a quantum proof hashing algorithm when the time comes which is fine.

However, everyone seems to gloss over the dead coins that will not be updated to these algorithms making them vulnerable. These coins (including satoshis) will most likely be stolen and dumped on the market crashing the price. (Governments will likely have incentive to do this as well.) I understand banks and every other software would be compromised, however, all other centralized softwares can upgrade once this vulnerability is discovered/exploited. My question primarily is focused on what happens with the dead addresses that we can’t upgrade.

I understand this won’t happen until at least 5-10 years from now, but knowing that the event WILL occur at some point does seem to be concerning. Can someone please explain why this is not a threat for a long term investor (my plan is to never stop DCAing).

UPDATE: please try to gear responses to the effect on bitcoin, not traditional banks or other institutions. They are centralized and will have updates in a matter of weeks as well can reverse transactions at their will. Bitcoin does not have this ability.

Second Update: SHA-256 is the algo used for protecting the network, not individual seed phrases. I understand that quantum won’t break the network, I’m specifically referring to private keys of dead coins.

Thanks!