Let's say the copyright period is not extended. Perhaps the government should then auction off the rights to Snow White, rather than putting it into the public domain. That is, Disney could buy a 25-year copyright extension, but they would have to pay through the nose for it.
I like the idea of being able to watch Snow White on Youtube for free, but I prefer the idea of significantly boosting cancer research.
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.
That kind of implies that everything we create inherently belongs to the government, "they" let us use it for the period of copyright, and then it's theirs again to sell off. I can make a case for copyright ending and creative works no longer having an owner, but I can't make a case (philosophically) for them to automatically belong to the government.
But in a democracy the government isn't "they", it is "we". If we all have the right to copy and distribute a particular creative work, then we can collectively decide to trade that right for something we find more valuable (such as cancer research).
I like the idea of being able to watch Snow White on Youtube for free, but I prefer the idea of significantly boosting cancer research.