well, when I refer to copyrights, I am generaly talking about things like artwork. In example, ID has released the code source for doom a long time ago, however, they haven't released the rights to the name "doom" as a video game.
code I sorta consider a non issue, I bet in the next decade, all software will end up being open source to anybody that wants access to it anyways. The reason behind my thinking is that there are only so many tasks that you could want automated based on the technology available at the time of its creation. E.G, there will only be so many FTP clients, so many desktop shells, so many game engines, etc, that sooner or later, the closed source ones wont be going anywhere because nobody has any need for them, as the open source ones tend to progress much faster than their closed source counterparts, until the closed source ones become obsolete. That said, long before the quake2 source code was released for example, there were already better open source 3d game engines, namely cube3d as an example. (and at this time, commercialy sold games would be based on how good the game artwork/play/fun actualy is, instead of the technology behind the game, because of course, they arent selling the code, only the story and the content of the game, or what have you. we also wouldn't see anymore crap like quake3 which was realy just a concept game, whose engine was to be used in better games.)
But (code aside, we are just talking about artwork here now) you want copyrights to be long enough to where theres a reason for somebody to want to create that copyright. Private individuals should have them last their entire lifetime, because its realy their art, they should be allowed to do what they want with it, and when they die it simply becomes public domain (or goes to their family members until 75 years after its creation, should they die before that timeframe). Corporate entities however, are different. Corporate entities are owned by multiple people, and their shares are for trade on the public market. Their copyrights, if you will, lack the "sovereignty" (for lack of a better term) that non corporate copyrights have, because while these copyrights were being created, they werent necessarily owned by the same people as when they were finnished.
/me tries to emphasize what hes saying here
Say for example, some employees work for a company and help create a video game, then all of them leave the company after x years. Now virtualy nobody who actualy did the work on that project actualy even works there anymore, yet the people who are in that company now own that copyright, even though they did nothing to create it.
heres my reasoning for the durations:
75 years works for individuals because its the average lifespan of the person.
25 years works for comanies because its the typical duration that a lifer would work for a company before retirement, so by this time, most people who took part in creating this copyright have left the company, hence the property of this publicly owned company goes to the public domain.
EDIT: Actualy heres a better idea, I know all of you will love this too - the companies copyright lasts only as long as their current layoff rate. Meaning, if employees are fired or laid off a lot due to corporate downsizing, like some CEO trying to save an extra buck, the company loses out because their copyrights will expire sooner! Only fat cats would hate a law like that, because now they actualy have to think before getting rid of people.