The usual way to do that is to have renewals or other periods; then things that are abandoned fall out of copyright, but things that the author is alive to protect remain in.

It's moderately hard to build a law based on what people think is "fair" mainly because fairness often has more to do with feelings (it would be fair for someone to make a Hobbit movie because the author is long dead; it would be unfair for someone to make a Potter movie because the author is alive, etc) than with an easily quantifiable rule.

I've often thought the solution is to define copyright (of things published, not trade secrets and unpublished works) as being something that can ONLY be defended as long as the work is "available" in the marketplace for "reasonable" amounts. As long as Warner Bros or whoever it is keeps selling the Lord of the Rings (extended edition) on DVD or whatever, they can j'accuse infringers of downloading it.

But ten years after it's no longer in print? No longer in copyright, either.