I'm well aware of the historical precedent, but that number was rather arbitrary even then - it was what a bunch of people agreed upon, based on their ideas and experience, and given the environment. It's doubly arbitrary today, considering how much the environment has changed. Is a 14-year copyright on software reasonable, for example, or too long.

Rather than making it a hard cut-off point, it would be interesting to come up with a scheme that attempts to capture the spirit of term limits.

Consider: why are copyright terms even a thing? Well, copyright is a monopoly on a thing that is not naturally restricted; it does not exist in the absence of society, and is therefore a privilege granted by that society. By itself, copyright is meant to encourage creativity in the interest of public good, and at the same time, to provide some means to derive profit from one's creative expression. So there are two conflicting interests at play here - the desire of the creator to be rewarded for the fruits of his labor, and the desire of the society to enjoy growing, constantly enriched culture. The copyright term, then, marks the point at which the latter trumps the former.

Instead, what we could do is capture the fact that the interests conflict. For as long as you hold copyright, you're effectively denying society the ability to freely enjoy the culture that you have enriched. Why, then, not tax the copyright accordingly? You could consider it a kind of intellectual property tax, but with a twist: the longer copyright is held, the more the interests of society are infringed, and the larger the compensatory payment required to maintain the copyright.

So we could start with a grace period of a couple of years that is completely free, then it starts growing steadily. For some really popular work that makes significant profits, the author could easily afford payments to maintain copyright for a decade or two (or however long - that is something that can be dialed arbitrarily). For things that are too obscure, payments would cease shortly, and they would fall to public domain. There wouldn't be such a thing as "abandonware" anymore.

What use to put the money to? Many possibilities there. Publicly sponsored arts and art education is an obvious choice. Another interesting example would be offering bulk sums of money to authors of culturally important works to surrender their copyrights sooner, so that the public can enjoy them.