The siblings have covered a lot of the issues with your suggestion. Wikipedia's page on philosophy of copyright might be informative as well. Other common arguments include the tragedy of the anticommons (having rightsholders for everything means doing anything new requires negotiating with too many different rightsholders) and the general fact that essentially all creative works build on prior creative works in some way, either direct retellings like many of Disney's movies or more indirectly like many fantasy books have elves that look a lot like those in Tolkien's Middle Earth.
There is also the complication that IP covers a lot of different things. Particularly I think there is a difference between artistic works like a novel and utilitarian works like Windows (and that there is not necessary a clear line between the two), but they are both covered by copyright under the exact same terms. Having copyright act differently for different works sounds messy and should probably be avoided in order to keep the law sensible, but both types of work have to be considered when arguing for how copyright should work.
For artistic works, the idea is that any published work is part of the collective culture and anyone should be able to build on it... with the exception that the author should have a limited monopoly on it in order to make money off of it. By having that time get too long, you get absurdities like the copyright status of the song "Happy Birthday to You" where the song has become part of American culture.
For utilitarian works, I think the argument might be closer to patents: the government wants to give some protection to new inventions in order to ensure a profit motive for developing them, but other companies should have access to old inventions in order to build on them. This doesn't quite work with software because there is no requirement tor release source code in order to get copyright on software. Of course, binaries alone can be useful and with effort can be modified to some extent if necessary.
The correct time-frame for both of those arguments is subjective and may be different, so the number that appears in copyright law should be a compromise between the two.