I mean, how does my suggested of using a cold wallet as an alternatively reliable option in a Post-Quantum World off-topic?
You did not understand this:
If bitcoin private key is no more safe with quantum computing and the public key of the private key is exposed, regardless of the wallet that you are using, the private key can be known as long as the public key is known. OP is very correct and adding cold wallets is off-topic tonehaybhenis discussing.
Let me be more specific.
If you have a cold wallet, and you use it to generate seed phrase, keys and addresses and no one knows your seed phrase nor private. In the post quantum world, if you make transaction, the public key of the address you used to make the transaction will be known on blockchain which quantum computer only needs to brute force your bitcoin address
private key. That is the reason it is advised that people should not reuse bitcoin address this time.