It doesn't just benefit one corporation. It profoundly benefits one corporation, and significantly benefits everyone else making a living off intellectual property. The longer the copyright term is, the greater the value of a copyright is.
The value of a copyright isn't really part of the equation though (I know this is orthogonal to your point). From the Constitution (emphasis mine):
"[Congress shall have the power] to promote the Progress of Science and the useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries."
Further, from Fox Film v. Doyal (again, emphasis mine):
"The sole interest of the United States and the primary object in conferring the [copyright] monopoly lie in the general benefits derived by the public from the labors of authors."
Copyright terms come down to a simple metric: if the public would benefit more from a work being in the public domain, the work shouldn't be copyright (insofar that the progress of science and innovation is not stifled). Corporations and anyone making a living off intellectual property have nothing to do with it; copyright exist only for the benefit of the general public. And with some of the most innovative systems software of the last decade being developed for free, you're going to have a hard time arguing that such lengthy copyright terms are needed to ensure that innovation happens. Linux is billion dollar project that happened organically; I just don't buy it that ~100 year copyright terms are necessary.
> significantly benefits everyone else making a living off intellectual property.
Doesn't that presuppose that the incremental gain from adding another N years of copyright coverage after an author's death is greater than the potential gain if it was accessible now to people who are making their living or--even--simply living?
It's not unreasonable to assume that an extremely large proportion of creators (who are only a small--but potentially growing--portion of society) will gain no real benefit from their works being covered by copyright for 70-100 years after their death but are negatively affected by everyone else's creations being covered in the same way.
It's not just a question of profit--it's also a question of culture. What wonderful and ground-breaking reinterpretations of culture from the 20th Century are never created--or illegal--because of current copyright law?
The date of an author's death has little to do with the value of her creation at the time of its creation. Copyrights are transferable. This business about death is a red herring.
Excluding work-for-hire, does a copyright transfer affect the copyright term in a way that makes the date of the original author's death no longer relevant? (IANAL and couldn't find a discussion of this when I had a quick look.)
Ignoring the impact of author death still doesn't change my original question of whether a longer term "significantly benefits everyone else making a living off intellectual property" and society in general.
This isn't complicated. A copyright is worth more if it lasts longer. An author can sell her rights to a work for more money if copyright is going to protect that work for a longer period of time. People making a living off intellectual property can be expected to make a better living in a world in which copyrighted works have more economic value.
The stuff about death + N is an attempt to tie things to an intuition of moral rights; the business case is actually pretty poor in general. The net present values of an asset leased for (say) 30 years, versus 130 years, are surprisingly close in value, not least because of the uncertainty in predicting the future.
That's easy: the patent covers the whole idea and precludes whole avenues of further exploration. Copyright only covers a single expression of an idea.
The assertion that the longer a copyright term is, the greater its value, seems dubious to me. It sounds plausible in isolation, but when you consider that every new idea builds on thousands of older ideas, longer copyright terms would seem to me to make copyrights less secure owing to challenge by antecedent rightsholders.
