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.