Well, what did the law state when the early works of Beatles were created?
The (theoretical) idea of a long copyright period is to encourage creativity. But what's the point of extending the copyright of that which has already been created? Extending periods retroactively will only stifle creativity, since creative reuse is hindered - while what has already been created is already there. From that point of view, both Disney movies and early Beatles songs should be in the public domain by now.