There are analyses that say many contradictory things. I haven't read that analysis and don't know whether I would think much of it. But let's suppose that if I did then I would be convinced that the economic effects of copyright extension are better understood as that type of protectionism.
That doesn't change the fact that I still have evidence that Disney is explicitly involved both directly and indirectly in lobbying for copyright extension. And they start lobbying for it like clockwork as soon as Steamboat Willie starts threatening to slide into public domain.
Therefore no matter what the economic arguments, I believe that the people actually changing the rules are changing it to keep Mickey Mouse under copyright.
That said, I think little of the position taken in that analysis. It claims that the public is forced to buy potentially-inferior modern IP. To the contrary, I believe that the public is free to not buy IP at all, but instead to find other ways to entertain themselves. In fact I believe that if modern authors and artists could freely draw on iconic past IP, they could create compelling new works drawing on the old ones which would increase consumption of IP. Thereby making the public happier AND content creators richer.
Therefore I believe that the net effect of copyright extension is to increase the rent that current content owners can get from past content at the cost of both the public and current content creators. (And yes, this holds true even though there are content creators employed by content owners.)
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As to "nobody benefits much", I remember one of the briefs that I saw for Eldred v. Ashcroft that demonstrated that over 99% of the possible present value of a work is captured by the author under the terms of the 1976 law. That is, if we take the possibility of public domain away and assign perpetual copyright to authors, we will not add more than about 1% to the compensation authors can expect.
Turning that figure around, today's content creators would be better off with the 1976 laws if they in aggregate could increase consumed commercial content by more than 1% from having full access to past content to work from. (Exactly like Walt Disney did with works like Sleeping Beauty.) I believe that this is quite possible. A number of economic experiments have found that increasing availability of free content increases consumption and ultimately increases revenue to artists. See, for example, http://www.baen.com/library/prime_palaver1.asp for one of them.