Michael Waldman (Cornell) presented a provocative seminar today at Melbourne Business School on copyright extensions and their social value. You can access the paper here. The paper related to the optimal length of copyright protection and the main idea was that copyrighted material could be subject to continual on-going investment (to maintain freshness and relevance) and so longer copyright length could encourage that in a socially useful way.
Now in terms of the 20 year copyright extension, all of the meat of potential upside was in the incentives on those whose copyright was just about to expire. Waldman cited the example of Mickey Mouse for which there had been little programming for 15 years prior to the 1998 extension and then lots thereafter. However, it seems to me that those things that are still capable of being invested in at 75 years when an unanticipated extension comes are just the sort of thing that could have been invested in without the copyright extension. Those that needed the extension were likely long-since dead. Thus, the material selected may have been precisely the material that needed the extension the least from a social perspective.
Nonetheless, the extension itself provides an interesting experient to see if the Mickey Mouse example generalises and was common on other copyrighted material.