It is dangerous to introduce a mandatory migration because it is a precedent and it significantly lessens the credibility of Bitcoin and the bit of our community that feels safe with Bitcoin.
I don't see anyone proposing a "mandatory migration", nor can I see how this is possible in practice. You can't force people to migrate to quantum-safe addresses. You can only point them how to do it, after a soft fork is passed. The "dilemma" is whether to freeze coins that do not migrate after year 20XY, or leave them in their fate. I would find many reasons why the former would result in a much worse network that people would lose trust to. In either case, people who do not get informed about the emerged threat, years into the future, will lose access to their coins, in either case, frozen or not frozen.
Am I missing something or did you not read the original post? That is the key point of the whole proposal. You can't force someone to migrate but by disallowing the spending of previous signature types you are in practice doing just that.
Phase B
- A deadline will be announced after which it won't be possible to spend and sign using legacy ECDSA/Schnorr signatures.
- This requires a consensus rule change, where nodes will reject the old signature formats.
- Quantum-vulnerable UTXOs become unspendable.
- The recommended deadline is around 5 years after the activation of Phase A.