I think this would be more compelling if you demonstrate that 7 years is long enough to generate 80% of the revenue. For many artistic works, it can take years to produce them (writing a book for instance), and many don't gain traction for years either. Film adaptations of books are a great example.

A real-life example: The Notebook (the book) was published in 1996. The Notebook (the movie) was released eight years later, in 2004. With a seven year copyright, the book would be in public domain, and the author may not see a dime from the movie. Is that fair? Is paying the author of the book royalties from the film's revenue a burdensome cost to society? I assert that the author of the book should be able to earn money from the wildly successful film adaptation of his movie, whether he's directly involved with the production of the film or not.

I'm sure there are a lot of subtle technicalities I'm missing, but I think the core of this argument stands.

I'm in favor of reducing the duration of copyright, but I think something in the region of 30 years is a more reasonable amount of time than 7 years.

Regarding the constitution, I think that the means of promoting creation of new works is the ability for creators to profit off of their works. By reducing the potential for profit, you reduce the incentive to create. I can see how overly-restrictive copyright law can inhibit creativity, but I think a 7 year period swings too far in the opposite direction.