I'm gonna use mainly economic arguments i think.
Yeah, I think this is the right decision, the question sounds like it's focusing on economics. I guess it doesn't make any difference now, but I would have mentioned these papers that set the economically optimal copyright term at 15 years:
Pollock, Rufus (2007) Forever Minus a Day? Some Theory and Empirics of Optimal Copyright.
Pollock, Rufus (2009) Forever Minus a Day? Calculating Optimal Copyright Term.
In order to best ensure economic prosperity, then, according to these papers, we ought to set the copyright term at 15 years.
I seem to recall something about 'fair use' contributing to the economy as well.
All of the empirical work in this area has come basically to the same conclusion--that patents are a particularly important inducement to invention in only a few industries. In pharmaceuticals, for example, patents seem to be an important part of the inducement for R&D. However, in industries like semiconductors and computers, the advantages that come with a head start--including setting up production, sales and service structures, and moving down the learning curve--were judged much more effective than patents as an inducement to R&D. In some of these industries, the respondents said that imitation was innately time-consuming and costly, even if there were no patent protection. In others, it was said that technology was moving so fast that patents were pointless. In any event, the empirical literature on appropriability certainly shows that there appear to be some industries where patents play a much smaller role than other forces in shaping the pattern of innovation. When we are looking at patent policy, we have to do so within the context of understanding how means other than patents induce invention and related activities. These other means include government grants and contracts, strong first-mover advantages, and rapid technological change.
ABA Section of Antitrust Law, Intellectual Property and Antitrust Handbook (2007), p. 108.