I'm not even certain why an end to copyright would mean an end to professional entertainment. It's not like professional entertainers did not exist before the 18th century, after all. Copyright is itself a product of the late early modern era, and people from Aristophanes to Shakespeare, and plenty both before and after, have made their living from entertainment without copyright. Besides, ruling that copyright infringement is protected speech under the First Amendment won't matter in the vast majority of cases; that crime is a matter for civil courts, and violators won't be taken to court by the various governments. Fun fact - private entities, such as those that hold most copyrights and would bring a suit against any violation of copyright law, can restrict First Amendment rights all they like. For instance, if I started spouting obscenities and ended up banned from these forums, I would have no cause to sue Toady One for violating my freedom of speech. This amendment would only block the government from protecting their copyrights.
It's also worth noting, on the other side of matters, that you don't even need to eliminate copyright (either directly or by a backdoor). There are plenty of intermediate reforms that could be made to make copyright law much more fair and equitable, especially when compared to patent law, without completely eliminating it.