You could just make it more and more expensive to extend copyright.<p>Ie first twenty years are free, the n-th year after will cost you some multiple of n USD.  (Or make it grow quadratic or exponential.)<p>Or you can get a fair valuation pretty easily: let Disney post a value for Mickey Mouse they&#x27;d want to be taxed on.  Lower value means less tax.  But the self-declared value comes with the obligation to sell to any comer who offers that much in cash.