That's the theory. The reality is that the new extended copyright terms well beyond the lifetime of the author mainly benefit corporate interests, and that's even before the law of unintended consequences kicks in. One of those consequences is that the new law makes it more and more difficult to reprint or adapt the work of deceased authors -- the rights situation is too complicated to make it worth the effort, given the modest returns such projects generate. Increasingly, work that goes out of print is pretty much guaranteed to stay out of print.
This is summed up in this definition by blogger and veteran editor Teresa Nielsen Hayden:
Copyright: During the author’s lifetime, the author’s nearly inalienable legal right to ownership of his or her own work. Thanks to the new extended terms of copyright (brought into law via means that would make a sausage-maker turn pale), the legal right of the author’s second ex-wife’s grandchildren by her third marriage to display their ignorance, exercise their greed, and gratify their egos to such an extent that they make the author’s work impossible to publish, adapt for theatrical performance, or excerpt in works of literary criticism, thus guaranteeing its permanent posthumous obscurity.Books die, and their life expectancy is getting shorter. Copyright law is one factor, but not the only one. You'll find an eye-opening discussion of this literary life expectancy shrinkage in Teresa's earlier post "The life expectancies of books," along with the extended comments.