Still, this seems rather easily solvable to naïve little old me.
20 year copyright by default, with a 20 year review cycle process an individual/company can apply to to ask for extension, on the condition that they can prove harm to newly generated IP, if the copyright is not-renewed. Still using Mickey Mouse to generate new works of non-derivative IP, and copyright of Mickey Mouse isn't causing damage to any other competing agencies (the way holding IP to a new drug or invention would)? Fine, renew granted.