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.