Also worth pointing out is the fact that length of time one retains a right to intellectual property influences how many works are created.

Handing out rights for 70 years for literary work probably doesn't get us any more benefit than limiting them to 25 years. Writers don't really decide whether to create based on such long term considerations, so why should we give it away for nothing in return?

On the other hand, if we make the period too short, a lot fewer works would be created.