So under your system, big blockbuster productions would have the longest copyright term.

I'd like to try exactly the opposite: when a work has been seen by a million people, it becomes public domain — whether that takes one year or 100 years.

Why? Because when you make a derivative work of Harry Potter or Star Wars you are not thinking that you can hoodwink your audience into believing that yours is the original, rather you're writing for a cultural context that assumes that people know the original. And to me that's exactly the underlying idea behind fair use.