LONG BEFORE before any actual Quantum Algorithm AND Quantum Computer exists that would allow someone to access the bitcoins secured by P2PKH, Bitcoin will already have moved to a quantum-resistant authorization algorithm. This will prevent Case 3, prevent the need for Case 2, and will allow plenty of time for any users that are still able to access their bitcoins to move those bitcoins with a quantum-resistant transaction. The coins that are still remaining in the weak transaction outputs once Quantum Technology becomes a realistic threat will be those coins that are effectively "lost". The QC owners will become the new owners of those coins, and Bitcoin will carry on as it always has.
It is very interesting. If a quantum-robust authorization algorithm protects bitcoin, will altcoins be able to protect themselves from the capabilities of quantum computers? If bitcoin remains invulnerable to them, then it is possible that the advent of quantum computers will even be useful for bitcoins in the sense that it will be possible to return to circulation over six million bitcoins that are now irretrievably lost.
However, I still fear the advent of quantum computers. This will provide additional opportunities for all kinds of hackers and scammers.