I believe this is why Phase C was proposed:


This phase is designed to preserve trust in Bitcoin by ensuring that users don't permanently lose access to their funds.
I think phase C should be mandatory rather than optional, as it provides a safeguard that maintains the network's trustworthiness while addressing quantum vulnerabilities.

I support Phase B and agree with this statement:



I believe that keep having both quantum-vulnerable and quantum-safe scripts could seriously undermine Bitcoin's overall trustworthiness.
If malicious actors with access to quantum computers steal coins, even with various blockchain analysis tools available, none of them perfectly prevents launderings, and attackers would likely succeed in laundering the stolen funds.

This could lead to Bitcoin losing public trust - creating a lose-lose situation for both society and the Bitcoin community as a whole.
There has been some academic research conducted on signature transition procedures, and long term unforgeability of signature is obviously one of the critical requirement of persistent blockchain.

[1] M. Sato and S. Matsuo, "Long-Term Public Blockchain: Resilience against Compromise of Underlying Cryptography," 2017 26th International Conference on Computer Communication and Networks (ICCCN), Vancouver, BC, Canada, 2017, pp. 1-8, doi: 10.1109/ICCCN.2017.8038516
[2]L. Meng and L. Chen, "An Enhanced Long-term Blockchain Scheme Against Compromise of Cryptography," Cryptology ePrint Archive, Paper 2021/1606, 2021.