One of the assumptions is that everything being equal the same works would exist if it were not for copyright protections. However, I would argue without the extended copyright protections, most of these [future] classic works would not exist, simply because publishers/studios would not invest in the creation/distribution of the works initially. In other words, copyright protections encourages the creation of works.
The OP takes an opposing stance, suggesting if copyright protections existed historically it would have stifled the creation of many classic works. This may be the case in certain instances, but to make that argument one must have an in depth understand of what constitutes copyright infringement in a legal sense - including all defenses to infringement (i.e. derivative work, fair use, educational/news worthy use, ect...)- and make the argument on a case by case basis. Very few people have any idea of what constitutes copyright infringement - and even among legal scholars, practitioners and judges there is disagreement.
All I know is if you have ever created anything and had it stolen you understand the need for legal protection. Plus it would suck to live in a world where I am financially rewarding thieves because I can not distinguish if a work was original or a knockoff. Finally, legal protection is just that protection, there is nothing stopping copyright owners from giving away their works for free, in other words voluntarily releasing their work(s) to the public domain.