A longer copyright validity period ensures that creators receive fair compensation for their original creations. In the United States, as of 2012, the copyright validity period is set at the lifespan of the creator plus an additional 70 years. This ensures that the original creator has time to profit from the creation and also ensures that his heirs can profit instead if he dies early or if the work does not become profitable until some time in the future.
Knowing that an original work will be copyrighted for a longer period of time, giving the creator more of a chance to profit from the work, encourages creativity. There is a risk inherent in creating new works rather than following an existing paradigm. For example, a longer copyright term encourages playwrights to create new plays, rather than continuing to perform plays that already exist, based on the possibility of making a significantly larger amount of money on something of the creator's own authorship.
Conversely, a longer copyright validity period also tends to stifle creativity. Works of the mind are nonrivalrous, meaning that ownership of the work creates a monopoly that is extremely difficult to maintain. A person who owns a car cannot prevent another person from buying the same or a similar car. A copyright, on the other hand, purports to prevent people from creating derivative works or expanding on a creative idea for a length of time so the creator can profit. That is a hard thing to do in practice, because once a person's mind absorbs the creation, there is no effective way to remove that knowledge from a person's thinking. A longer copyright period becomes unsustainable because it creates an artificial barrier to what the human brain does naturally: expound, innovate, create and re-create.
Burden on Society
Allowing longer copyright validity periods places an extraordinary burden on society. Having to obtain permission to use works of the mind affects the arts, education and innovation. While this burden is reasonable for the time it takes to fairly compensate the creator, there is a line beyond which copyright protection burdens society more than it benefits creators. For example, if the works of Shakespeare were still under copyright, the effect on schools, arts organizations and other creators who have used Shakespeare's works as the basis for a new work would be staggering. The heirs of that legendary author would benefit but society would have lost an immeasurable treasure trove of derivative creativity.