That would require the government to take ownership of Snow White - that is, it would require that after a certain amount of time, all copyrighted works become the property of the government, who may then be under some mandate to auction it off or whatnot.

Maybe some folks would be okay with this, but I would much rather my creations be owned by no one after a certain period than that the government just exercises some form of eminent domain over my property to take possession of it.