I think an even more simple and self limiting solution would be to require a payment for copyright that doubles every period.

For example, the first 5 years would be $100, the next 5 would be $200 ... by year 100 you're paying $52.5 million to extend your copyright by another 5 years.

You could even set up a scheme where you can pre-pay your copyright dues to a set period. Want 30 years w/o having to renew, put down $5,600 up front and it's good. Want to test the waters on something and lock in for 5? Send in $100 and let it go abandoned 5 years later if things don't pan out.