I don't see why. I don't see why should they have the exclusive right to sell it anymore, and sue anyone that creates a derivative work based on their work older than 10 years.

Also Girl Genius still would have a lot of options to make money of it. I just think derivative work should be able to appear, and more than 10 years old stuff should be free as in freedom and as in free beer. And we, as a society could choose this by modifying our laws.