= Rather than fight to reduce the maximum term of copyrights, consider that existing and future copyrights could be taxed annually just like real estate as long as they are kept from the public domain. This uses a market-based approach to limit the external costs of copyright monopolies. 
"What is the social justification for such a tax?
Real property taxes are justified by the notion that real estate imposes a cost on society -- for fire departments, police departments, schools, roads, sewers, water pipelines, libraries, town courts, property record archives, and so forth.
Copyrights were originally monopolies granted "for a limited time" with the notion that the costs they imposed on society would be repaid by the work moving into the public domain after that limited time. That bargain has effectively been broken because the terms are so long (and likely will be in perpetuity in the U.S.A. given the recent Supreme Court decision). Yet, copyrights still pose a cost on society. There must be courts to dispute them, police to enforce them. There must be prisons to hold the millions of copyright offenders. Like no one in the 1960s would imagine a million U.S. citizens behind bars for non-violent drug offenses in the 1990s, it is possible that there may be a million U.S. citizens behind bars in the 2010s for copyright violations as the "War on Those Who Share" gets underway. There must be an information superhighway to transport these works, and standards for disseminating them. Authors of derivative works must spend time researching whether a work is already in the public domain, or locating all the related rights holders if it is not. Extensions of the principle of copyright to cover the ideas in the work such as characters or plot lines or other structures make it ever more costly to create new non-infringing works. Many new or derived works are not created because of these chilling effects, which is a hidden cost of copyrights. People in developing nations or others who cannot pay use fees for copyrighted works are deprived of education or enjoyment when such a deprivation does not directly benefit anyone. So, given all these indirect costs of granting copyright monopolies, society is justified in imposing a financial cost on copyright holders to rebalance the copyright bargain.
Real estate is typically taxed at a small percentage of an assessed value. If the taxes are not paid, the real estate essentially becomes owned by society. Note that these annual property taxes are in addition to any fees for recording deed transfers, liens, title searches, and such.
Since it is difficult to value a copyright, one possibility to determine the value of a copyright is to let copyright holders assess themselves how much it is worth it to them to keep their work out of the public domain. Then the rights holder would pay annually a small percentage of this value (perhaps three to five percent). Each year, when the rights holder sent in their tax, the rights holder could change this self-assessed value to reflect their changing priorities and a changing market. If the rights holder did not pay the tax, then the work would move immediately into the public domain. If someone wanted that work in the public domain, they could pay the copyright holder the self-assessed amount and the work would then immediately be moved into the public domain. This public domain buyout possibility serves to limit the tendency of rights holders to produce low self-assessments to minimize their annual tax payments.
This approach could include a digital archive of all copyrighted works. Essentially, upon initial registration of a self-assessed value, a rights holder would be required to send in a digital copy of the work. This copy would be used to determine rights holders for works by means of a digital search. Any work not in this database would be presumed public domain. If the annual tax were not paid, this archive would be a place to get a copy of the now public domain work.
This approach could also be scaled internationally where a copyright holder would have to determine a self-assessed amount in each country for the privilege of having their copyright enforceable in that country. A benefit of this approach is that developing nations could gain a nice income from such monopoly payments, rather than simply suffer the knowledge of having a citizenry deprived of the fruits of human achievement.
Incidentally, this approach can be applied to patents as well, although since patents are much more limited in scope, the societal bargain has not been so badly broken for their monopolies." (http://journalism.berkeley.edu/projects/biplog/archive/000431.html)